Properly detect timeout in pthread_mutex_lock_timeout_np_impl

Previously we were checking against a positive errno which
would not be returned from a system call.

Change-Id: I8e3a36f6fbf5ccc2191a152a1def37e2d6f93124
Allocate additional space on stack for indirect syscall

The caller is only required to allocate 16 bytes on the
stack for a0-a3. syscall is handling up to 6 arguments so
additional space is needed on the stack to avoid corrupting the
callers frame.

Change-Id: I054b31696decc3e17d9c70af18cd278b852235d1