[C2x] Implement support for revised spelling of keywords

This implements WG14 N2934
(https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2934.pdf), which
adds keywords for alignas, alignof, bool, static_assert, and
thread_local in C, as aliases for _Alignas, _Alignof, _Bool,
_Static_assert, and _Thread_local. We already supported the keywords in
C2x mode, but this completes support by adding pre-C2x compat warnings
and updates the stdalign.h header in freestanding mode.