[libc] move pthread macros to dedicated header (#119286)
so that docgen can find our definitions.
Also eliminate the enums. POSIX is careful to call these "symbolic constants"
rather than specifically whether they are preprocessor macro defines or not.
Enums are useful to expressing mutual exclusion when the enum values are in
distinct enums which can improve type safety. Our enum values weren't using
that pattern though; they were all in one big anonymous enum.
Link:
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/pthread.h.html
Fixes: #88997
