[libc] Add POSIX functions posix_spawn_file_actions_*.

Namely, posix_spawn_file_actions_addclose,
posix_spawn_file_actions_adddup2, posix_spawn_file_actions_addopen,
posix_spawn_file_actions_destroy, posix_spawn_file_actions_init have
been added.

Reviewed By: michaelrj, lntue

Differential Revision: https://reviews.llvm.org/D135603