pthread.c: add Mac OS X sched_yield() support

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
common.mk: account for Mac OS X flag needed to understand AVX

We don't emit AVX by default, but adding these flags allows users to specify
-mavx in CFOPTIMIZE.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>