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)