stddef: Define 'unreachable', for ISO C 23 compliance.
* lib/verify.h (_GL_HAS_BUILTIN_UNREACHABLE): Don't define if already
defined.
* lib/stddef.in.h (_GL_HAS_BUILTIN_UNREACHABLE, unreachable): New
macros.
(abort): Declare if needed for unreachable.
* m4/stddef_h.m4 (gl_STDDEF_H): Test for unreachable.
* tests/test-stddef.c (test_unreachable_optimization,
test_unreachable_noreturn): New functions, based on tests/test-verify.c.
* doc/posix-headers/stddef.texi: Mention unreachable.