Update supportsUnaligned in xor.go to be true for
GOARCH values ppc64le and ppc64.  This allows the
xor of long buffers to be done on double words
(8 bytes) instead of a single byte at a time, which
significantly improves performance.

Fixes #14350

Change-Id: Iccc6b9d3df2e604a55f4c1e4890bdd3bb0d77ab0
Reviewed-on: https://go-review.googlesource.com/19519Reviewed-by: Minux Ma <minux@golang.org>