powerpc: Revert to default atomic ops in elision code

Power ISA 2.07B section B.5.5 relaxed the barrier requirement around a
TLE enabled lock.  It is now identical to a traditional lock.

2015-08-26  Paul E. Murphy  <murphyp@linux.vnet.ibm.com>

	* sysdeps/unix/sysv/linux/powerpc/elision-lock.c
	(__arch_compare_and_exchange_val_32_acq): Remove and use common
	definition.  ISA 2.07B no longer requires full sync.

(cherry picked from commit 6eb901de)