extend.texi: Fix up defbuiltin* with spaces in return type

In https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fstdc_005fbit_005ffloor
I've noticed that while e.g. __builtin_stdc_bit_floor builtin is properly
rendered in bold and bigger size, for the __builtin_stdc_bit_width builtin
it is not the builtin name which is marked like that, but the keyword int
before it.  Also, seems such builtins are missing from the index.

I've read the texinfo docs and they seem to suggest in
https://www.gnu.org/software/texinfo/manual/texinfo/html_node/Line-Macros.html
that return types of functions with spaces in the return type should be
wrapped with {}s and we already use that e.g. in
@defbuiltin{{void *} __builtin_thread_pointer (void)}

The following patch adjusts builtins I found which contained one or two
spaces in the return type name (plus two spots which used 2 spaces after
single keyword return type instead of 1 which triggered my search regex as
well).

2023-12-01  Jakub Jelinek  <jakub@redhat.com>

	* doc/extend.texi (__builtin_darn, __builtin_darn_raw,
	__builtin_ia32_vec_ext_v2di, __builtin_ia32_crc32qi,
	__builtin_ia32_crc32hi, __builtin_ia32_crc32si,
	__builtin_ia32_crc32di): Put {}s around return type with spaces in it.
	(__builtin_rx_mvfachi, __builtin_rx_mvfacmi): Remove superfluous
	whitespace.

(cherry picked from commit ff99671a)

gcc/doc/extend.texi

@@ -18415,8 +18415,8 @@ round to odd as the rounding mode.
 The following additional built-in functions are also available for the
 PowerPC family of processors, starting with ISA 3.0 or later:
-@defbuiltin{long long __builtin_darn (void)}
-@defbuiltinx{long long __builtin_darn_raw (void)}
+@defbuiltin{{long long} __builtin_darn (void)}
+@defbuiltinx{{long long} __builtin_darn_raw (void)}
 @defbuiltinx{int __builtin_darn_32 (void)}
 The @code{__builtin_darn} and @code{__builtin_darn_raw}
 functions require a
@@ -21306,12 +21306,12 @@ multiplying the bottom 16 bits of the two arguments into the
 accumulator.
 @enddefbuiltin
-@defbuiltin{int  __builtin_rx_mvfachi (void)}
+@defbuiltin{int __builtin_rx_mvfachi (void)}
 Generates the @code{mvfachi} machine instruction to read the top
 32 bits of the accumulator.
 @enddefbuiltin
-@defbuiltin{int  __builtin_rx_mvfacmi (void)}
+@defbuiltin{int __builtin_rx_mvfacmi (void)}
 Generates the @code{mvfacmi} machine instruction to read the middle
 32 bits of the accumulator.
 @enddefbuiltin
@@ -22665,7 +22665,7 @@ Generates the @code{extractps} machine instruction.
 Generates the @code{pextrd} machine instruction.
 @enddefbuiltin
-@defbuiltin{long long __builtin_ia32_vec_ext_v2di (v2di, const int)}
+@defbuiltin{{long long} __builtin_ia32_vec_ext_v2di (v2di, const int)}
 Generates the @code{pextrq} machine instruction in 64bit mode.
 @enddefbuiltin
@@ -22694,19 +22694,19 @@ v2di __builtin_ia32_pcmpgtq (v2di, v2di);
 The following built-in functions are available when @option{-msse4.2} is
 used.
-@defbuiltin{unsigned int __builtin_ia32_crc32qi (unsigned int, unsigned char)}
+@defbuiltin{{unsigned int} __builtin_ia32_crc32qi (unsigned int, unsigned char)}
 Generates the @code{crc32b} machine instruction.
 @enddefbuiltin
-@defbuiltin{unsigned int __builtin_ia32_crc32hi (unsigned int, unsigned short)}
+@defbuiltin{{unsigned int} __builtin_ia32_crc32hi (unsigned int, unsigned short)}
 Generates the @code{crc32w} machine instruction.
 @enddefbuiltin
-@defbuiltin{unsigned int __builtin_ia32_crc32si (unsigned int, unsigned int)}
+@defbuiltin{{unsigned int} __builtin_ia32_crc32si (unsigned int, unsigned int)}
 Generates the @code{crc32l} machine instruction.
 @enddefbuiltin
-@defbuiltin{unsigned long long __builtin_ia32_crc32di (unsigned long long, unsigned long long)}
+@defbuiltin{{unsigned long long} __builtin_ia32_crc32di (unsigned long long, unsigned long long)}
 Generates the @code{crc32q} machine instruction.
 @enddefbuiltin