c: Add __builtin_stdc_* builtins

c: Add __builtin_stdc_* builtins

As discussed in the
https://sourceware.org/pipermail/libc-alpha/2023-November/152756.html
thread, including e.g.
https://sourceware.org/pipermail/libc-alpha/2023-November/152795.html
patch, while one can use the new __builtin_{clz,ctz,popcount}g builtins
to implement the stdbit.h type-generic macros, there are certain problems
with that implementation if those macros must be usable outside of
function bodies (e.g. int a = sizeof (stdc_bit_floor (0ULL));), must not
evaluate their arguments multiple times and especially for deep stdc_*
macro nesting don't expand the argument more than once.  Plus ideally are
usable in constant expressions for all the types if they have constant
arguments.  The above second URL satisfies it all but the last two (the
last one satisfies for many of them).  While we could get away with just
adding __biultin_stdc_bit_{ceil,floor,width} which are complicated and
2 further extensions (some way to say that __builtin_c{l,t}zg should
imply bit precision of the first argument for the second argument without
using __builtin_popcountg ((__typeof (x)) -1) in there because that
causes another expansion of the macro argument and say __builtin_bit_complement
type-generic builtin which would be like (__typeof (x)) ~(x)), it was decided
we want to implement builtins for all the stdc type-generic macros.
As we are close to running out of 8-bit enum rid (when adding the 14 new
RID_* we have 7 too many), this patch implements those 14 keywords using
a single RID_BUILTIN_STDC and simply in the rare case this is being
parsed check values of 1-2 characters from the builtin names to see which
one it is.

2023-11-23  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* doc/extend.texi (__builtin_stdc_bit_ceil, __builtin_stdc_bit_floor,
	__builtin_stdc_bit_width, __builtin_stdc_count_ones,
	__builtin_stdc_count_zeros, __builtin_stdc_first_leading_one,
	__builtin_stdc_first_leading_zero, __builtin_stdc_first_trailing_one,
	__builtin_stdc_first_trailing_zero, __builtin_stdc_has_single_bit,
	__builtin_stdc_leading_ones, __builtin_stdc_leading_zeros,
	__builtin_stdc_trailing_ones, __builtin_stdc_trailing_zeros): Document.
gcc/c-family/
	* c-common.h (enum rid): Add RID_BUILTIN_STDC: New.
	* c-common.cc (c_common_reswords): Add __builtin_stdc_bit_ceil,
	__builtin_stdc_bit_floor, __builtin_stdc_bit_width,
	__builtin_stdc_count_ones, __builtin_stdc_count_zeros,
	__builtin_stdc_first_leading_one, __builtin_stdc_first_leading_zero,
	__builtin_stdc_first_trailing_one, __builtin_stdc_first_trailing_zero,
	__builtin_stdc_has_single_bit, __builtin_stdc_leading_ones,
	__builtin_stdc_leading_zeros, __builtin_stdc_trailing_ones and
	__builtin_stdc_trailing_zeros.  Move __builtin_assoc_barrier
	alphabetically earlier.
gcc/c/
	* c-parser.cc (c_parser_postfix_expression): Handle RID_BUILTIN_STDC.
	* c-decl.cc (names_builtin_p): Likewise.
gcc/testsuite/
	* gcc.dg/builtin-stdc-bit-1.c: New test.
	* gcc.dg/builtin-stdc-bit-2.c: New test.