powerpc: Fix IFUNC for memrchr

Recent commit 59ba2d2b missed to add __memrchr_power8 in
ifunc list.  Also handled discarding unwanted bytes for
unaligned inputs in power8 optimization.

2017-10-05  Rajalakshmi Srinivasaraghavan  <raji@linux.vnet.ibm.com>

	* sysdeps/powerpc/powerpc64/multiarch/memrchr-ppc64.c: Revert
	back to powerpc32 file.
	* sysdeps/powerpc/powerpc64/multiarch/memrchr.c
	(memrchr): Add __memrchr_power8 to ifunc list.
	* sysdeps/powerpc/powerpc64/power8/memrchr.S: Mask
	extra bytes for unaligned inputs.