powerpc: Merge semaphore.h

Adopted the ppc64 version of semaphore.h.  The 32-bit version used
smp_wmb(), but recent updates to atomic.h mean this is no longer required.
The 64-bit version made use of unlikely(), which has been retained in the
combined version.

This patch requires the recent atomic.h patch.
Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Kumar Gala <kumar.gala@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>