[libc] Use the dup2 syscall wrapper in posix_spawn (#207879)

posix_spawn's local dup2 helper fell back to a bare dup3 on targets
without SYS_dup2. dup3 fails with EINVAL when oldfd == newfd, where dup2
must instead return oldfd if it is valid, so
posix_spawn_file_actions_adddup2 with equal fds misbehaved on those
targets. linux_syscalls::dup2 already implements the correct fallback,
so call it directly and drop the local helper.

GitOrigin-RevId: 3c7727f1269e5986915aacf4ee1ae117843bd573