x86: Cleanup pthread_spin_{try}lock.S

Save a jmp on the lock path coming from an initial failure in
pthread_spin_lock.S.  This costs 4-bytes of code but since the
function still fits in the same number of 16-byte blocks (default
function alignment) it does not have affect on the total binary size
of libc.so (unchanged after this commit).

pthread_spin_trylock was using a CAS when a simple xchg works which
is often more expensive.

Full check passes on x86-64.