[debug] Fix dwarf v4 .debug_macro.dwo

Consider a hello world, compiled with -gsplit-dwarf and dwarf version 4, and
-g3:
...
$ gcc -gdwarf-4 -gsplit-dwarf /data/vries/hello.c -g3 -save-temps -dA
...

In section .debug_macro.dwo, we have:
...
.Ldebug_macro0:
        .value  0x4     # DWARF macro version number
        .byte   0x2     # Flags: 32-bit, lineptr present
        .long   .Lskeleton_debug_line0
        .byte   0x3     # Start new file
        .uleb128 0      # Included from line number 0
        .uleb128 0x1    # file /data/vries/hello.c
        .byte   0x5     # Define macro strp
        .uleb128 0      # At line number 0
        .uleb128 0x1d0  # The macro: "__STDC__ 1"
...

Given that we use a DW_MACRO_define_strp, we'd expect 0x1d0 to be an
offset into a .debug_str.dwo section.

But in fact, 0x1d0 is an index into the string offset table in
section .debug_str_offsets.dwo:
...
        .long   0x34f0  # indexed string 0x1d0: __STDC__ 1
...

Add asserts that catch this inconsistency, and fix this by using
DW_MACRO_define_strx instead.

Tested on x86_64.

gcc/ChangeLog:

2024-05-14  Tom de Vries  <tdevries@suse.de>

	PR debug/115066
	* dwarf2out.cc (output_macinfo_op): Fix DW_MACRO_define_strx/strp
	choice for v4 .debug_macro.dwo.  Add asserts to check that choice.

gcc/testsuite/ChangeLog:

2024-05-14  Tom de Vries  <tdevries@suse.de>

	PR debug/115066
	* gcc.dg/pr115066.c: New test.