varasm: Add support for emitting binary data with the new gas .base64 directive

Nick has implemented a new .base64 directive in gas (to be shipped in
the upcoming binutils 2.43; big thanks for that).
See https://sourceware.org/bugzilla/show_bug.cgi?id=31964

The following patch adjusts default_elf_asm_output_ascii (i.e.
ASM_OUTPUT_ASCII elfos.h implementation) to use it if it detects binary
data and gas supports it.

Without this patch, we emit stuff like:
        .string "\177ELF\002\001\001\003"
        .string ""
        .string ""
        .string ""
        .string ""
        .string ""
        .string ""
        .string ""
        .string "\002"
        .string ">"
...
        .string "\324\001\236 0FS\202\002E\n0@\203\004\005&\202\021\337)\021\203C\020A\300\220I\004\t\b\206(\234\0132l\004b\300\bK\006\220$0\303\020P$\233\211\002D\f"
etc., with this patch more compact
        .base64 "f0VMRgIBAQMAAAAAAAAAAAIAPgABAAAAABf3AAAAAABAAAAAAAAAAACneB0AAAAAAAAAAEAAOAAOAEAALAArAAYAAAAEAAAAQAAAAAAAAABAAEAAAAAAAEAAQAAAAAAAEAMAAAAAAAAQAwAAAAAAAAgAAAAAAAAAAwAAAAQAAABQAwAAAAAAAFADQAAAAAAAUANAAAAAAAAcAAAAAAAAABwAAAAAAAAAAQAAAAAAAAABAAAABAAAAAAAAAAAAAAAAABAAAAAAAAAAEAAAAAAADBwOQAAAAAAMHA5AAAAAAAAEAAAAAAAAAEAAAAFAAAAAIA5AAAAAAAAgHkAAAAA"
        .base64 "AACAeQAAAAAAxSSgAgAAAADFJKACAAAAAAAQAAAAAAAAAQAAAAQAAAAAsNkCAAAAAACwGQMAAAAAALAZAwAAAADMtc0AAAAAAMy1zQAAAAAAABAAAAAAAAABAAAABgAAAGhmpwMAAAAAaHbnAwAAAABoducDAAAAAOAMAQAAAAAA4MEeAAAAAAAAEAAAAAAAAAIAAAAGAAAAkH2nAwAAAACQjecDAAAAAJCN5wMAAAAAQAIAAAAAAABAAgAAAAAAAAgAAAAAAAAABAAAAAQAAABwAwAAAAAAAHADQAAAAAAAcANAAAAAAABAAAAAAAAAAEAAAAAAAAAACAAAAAAA"
        .base64 "AAAEAAAABAAAALADAAAAAAAAsANAAAAAAACwA0AAAAAAACAAAAAAAAAAIAAAAAAAAAAEAAAAAAAAAAcAAAAEAAAAaGanAwAAAABoducDAAAAAGh25wMAAAAAAAAAAAAAAAAQAAAAAAAAAAgAAAAAAAAAU+V0ZAQAAABwAwAAAAAAAHADQAAAAAAAcANAAAAAAABAAAAAAAAAAEAAAAAAAAAACAAAAAAAAABQ5XRkBAAAAAw/WAMAAAAADD+YAwAAAAAMP5gDAAAAAPy7CgAAAAAA/LsKAAAAAAAEAAAAAAAAAFHldGQGAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
        .base64 "AAAAAAAAAAAAAAAAAAAAAAAAABAAAAAAAAAAUuV0ZAQAAABoZqcDAAAAAGh25wMAAAAAaHbnAwAAAACYGQAAAAAAAJgZAAAAAAAAAQAAAAAAAAAvbGliNjQvbGQtbGludXgteDg2LTY0LnNvLjIAAAAAAAQAAAAwAAAABQAAAEdOVQACgADABAAAAAEAAAAAAAAAAQABwAQAAAAJAAAAAAAAAAIAAcAEAAAAAwAAAAAAAAAEAAAAEAAAAAEAAABHTlUAAAAAAAMAAAACAAAAAAAAAAOAAACsqAAAgS0AAOJWAAAjNwAAXjAAAAAAAAAAAAAAF1gAAHsxAABBBwAA"
        .base64 "G0kAALGmAACwoAAAAAAAAAAAAACQhAAAAAAAAOw1AACNYgAAAAAAAFQoAAAAAAAAx3UAALZAAAAAAAAAiIUAALGeAABBlAAAWEsAAPmRAACmOgAAAAAAADh3AAAAAAAAlCAAAAAAAABymgAAaosAAMIjAAAKMQAAMkIAADU0AAAAAAAA5ZwAAAAAAAAAAAAAAAAAAFIdAAAIGQAAAAAAAMFbAAAoTQAAGDcAAIRgAAA6HgAAlxwAAAAAAADOlgAAAAAAAEhPAAARiwAAMGgAAOVtAADMFgAAAAAAAAAAAACrjgAAYl4AACZVAAA/HgAAAAAAAAAAAABqPwAAAAAA"
The patch attempts to juggle between readability and compactness, so
if it detects some hunk of the initializer that would be shorter to be
emitted as .string/.ascii directive, it does so, but if it previously
used .base64 directive it switches mode only if there is a 16+ char
ASCII-ish string.

On my #embed testcase from yesterday
unsigned char a[] = {
 #embed "cc1plus"
};
without this patch it emits 2.4GB of assembly, while with this
patch 649M.
Compile times (trunk, so yes,rtl,extra checking) are:
time ./xgcc -B ./ -S -std=c23 -O2 embed-11.c

real    0m13.647s
user    0m7.157s
sys     0m2.597s
time ./xgcc -B ./ -c -std=c23 -O2 embed-11.c

real    0m28.649s
user    0m26.653s
sys     0m1.958s
without the patch and
time ./xgcc -B ./ -S -std=c23 -O2 embed-11.c

real    0m4.283s
user    0m2.288s
sys     0m0.859s
time ./xgcc -B ./ -c -std=c23 -O2 embed-11.c

real    0m6.888s
user    0m5.876s
sys     0m1.002s
with the patch, so that feels like significant improvement.
The resulting embed-11.o is identical between the two ways of expressing
the mostly binary data in the assembly.  But note that there are portions
like:
        .base64 "nAAAAAAAAAAvZRcAIgAOAFAzMwEAAAAABgAAAAAAAACEQBgAEgAOAFBHcAIAAAAA7AAAAAAAAAAAX19nbXB6X2dldF9zaQBtcGZyX3NldF9zaV8yZXhwAG1wZnJfY29zaABtcGZyX3RhbmgAbXBmcl9zZXRfbmFuAG1wZnJfc3ViAG1wZnJfdGFuAG1wZnJfc3RydG9mcgBfX2dtcHpfc3ViX3VpAF9fZ21wX2dldF9tZW1vcnlfZnVuY3Rpb25zAF9fZ21wel9zZXRfdWkAbXBmcl9wb3cAX19nbXB6X3N1YgBfX2dtcHpfZml0c19zbG9uZ19wAG1wZnJfYXRh"
        .base64 "bjIAX19nbXB6X2RpdmV4YWN0AG1wZnJfc2V0X2VtaW4AX19nbXB6X3NldABfX2dtcHpfbXVsAG1wZnJfY2xlYXIAbXBmcl9sb2cAbXBmcl9hdGFuaABfX2dtcHpfc3dhcABtcGZyX2FzaW5oAG1wZnJfYXNpbgBtcGZyX2NsZWFycwBfX2dtcHpfbXVsXzJleHAAX19nbXB6X2FkZG11bABtcGZyX3NpbmgAX19nbXB6X2FkZF91aQBfX2dtcHFfY2xlYXIAX19nbW9uX3N0YXJ0X18AbXBmcl9hY29zAG1wZnJfc2V0X2VtYXgAbXBmcl9jb3MAbXBmcl9zaW4A"
        .string "__gmpz_ui_pow_ui"
        .string "mpfr_get_str"
        .string "mpfr_acosh"
        .string "mpfr_sub_ui"
        .string "__gmpq_set_ui"
        .string "mpfr_set_inf"
...
        .string "GLIBC_2.14"
        .string "GLIBC_2.11"
        .base64 "AAABAAIAAQADAAMAAwADAAMAAwAEAAUABgADAAEAAQADAAMABwABAAEAAwADAAMAAwAIAAEAAwADAAEAAwABAAMAAwABAAMAAQADAAMAAwADAAMAAwADAAYAAwADAAEAAQAIAAMAAwADAAMAAwABAAMAAQADAAMAAQABAAEAAwAIAAEAAwADAAEAAwABAAMAAQADAAEABgADAAMAAQAHAAMAAwADAAMAAwABAAMAAQABAAMAAwADAAkAAQABAAEAAwAKAAEAAwADAAMAAQABAAMAAwALAAEAAwADAAEAAQADAAMAAwABAAMAAwABAAEAAwADAAMABwABAAMAAwAB"
        .base64 "AAEAAwADAAEAAwABAAMAAQADAAMAAwADAAEAAQABAAEAAwADAAMAAQABAAEAAQABAAEAAQADAAMAAwADAAMAAQABAAwAAwADAA0AAwADAAMAAwADAAEAAQADAAMAAQABAAMAAwADAAEAAwADAAEAAwAIAAMAAwADAAMABgABAA4ACwAGAAEAAQADAAEAAQADAAEAAwABAAMAAwABAAEAAwABAAMAAwABAAEAAwADAAMAAwABAAMAAQABAAEAAQABAAMADwABAAMAAQADAAMAAwABAAEAAQAIAAEADAADAAMAAQABAAMAAwADAAEAAQABAAEAAQADAAEAAwADAAEA"
        .base64 "AwABAAMAAQADAAMAAQABAAEAAwADAAMAAwADAAMAAQADAAMACAAQAA8AAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQA="
so it isn't all just totally unreadable stuff.

2024-07-15  Jakub Jelinek  <jakub@redhat.com>

	* configure.ac (HAVE_GAS_BASE64): New check.
	* config/elfos.h (BASE64_ASM_OP): Define if HAVE_GAS_BASE64 is
	defined.
	* varasm.cc (assemble_string): Bump maximum from 2000 to 16384 if
	BASE64_ASM_OP is defined.
	(default_elf_asm_output_limited_string): Emit opening '"' together
	with STRING_ASM_OP.
	(default_elf_asm_output_ascii): Use BASE64_ASM_OP if defined and
	beneficial.  Remove UB when last_null is NULL.
	* configure: Regenerate.
	* config.in: Regenerate.