c: Add C2X BOOL_MAX and BOOL_WIDTH to limits.h

c: Add C2X BOOL_MAX and BOOL_WIDTH to limits.h

C2X adds BOOL_MAX and BOOL_WIDTH macros to <limits.h>.  As GCC only
supports values 0 and 1 for _Bool (regardless of the number of bits in
the representation, other bits are padding bits and if any of them are
nonzero, the representation is a trap representation), the values of
those macros can just be hardcoded directly in <limits.h> rather than
needing corresponding predefined macros.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/
	* glimits.h [__STDC_VERSION__ > 201710L] (BOOL_MAX, BOOL_WIDTH):
	New macros.

gcc/testsuite/
	* gcc.dg/c11-bool-limits-1.c, gcc.dg/c2x-bool-limits-1.c: New
	tests.