[libc][POSIX] provide WIFSIGNALED from <sys/wait.h>

[nickdesaulniers]

building llvm with llvm-libc fails with:

/llvm-project-main/llvm/lib/Support/Unix/Program.inc:497:14: error: use of undeclared identifier 'WIFSIGNALED'

we're missing this macro from POSIX

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

building llvm with llvm-libc fails with: ``` /llvm-project-main/llvm/lib/Support/Unix/Program.inc:497:14: error: use of undeclared identifier 'WIFSIGNALED' ``` we're missing this macro from POSIX

[nickdesaulniers]

We also need WIFEXITED, WEXITSTATUS, and WTERMSIG for #125092.

[nickdesaulniers]

WIFEXITED, WEXITSTATUS, and WTERMSIG

e3638e8

The underlying issue for those 3 is #125102.

WIFSIGNALED

c9783d2 dropped this by accident...