libc: Add prototypes for various functions we use in __weak_reference().

<sys/cdefs.h>: Rework __weak_reference() macro.

My LTO build blew away weak symbols from slim LTO objects.
Use __strong_reference() + weak attribute to allow the compiler to catch
extern declaration

<sys/cdefs.h>: Rework __weak_reference() macro.

My LTO build blew away weak symbols from slim LTO objects.
Use __strong_reference() + weak attribute to allow the compiler to catch
extern declarations and not to fold weak symbols as local ones.

Keep previous version as __weak_reference_asm() in _pthread_stubs.c for
now due to several issues (there is a need to do it in a cleaner way).

lib/libc/gen/ucontext.c:
add missing __DECONST for ucp, shouldn't sigreturn take const ucontext_t?

lib/libc/inet/inet_ntoa.c:
add missing #undef inet_ntoa_r

No symbol changes in libc and librt on normal compilation.

While there, add __weak_symbol attribute for future additions.

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libc: Fix warnings fallout from the change to CSTD=gnu99.

Implement sigwait.