lib/raid6: build proper files on corresponding arch

sse and avx2 stuff only exist on x86 arch, and we don't need to build
altivec on x86. And we can do that at lib/raid6/Makefile.

Proposed-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Jim Kukunas <james.t.kukunas@linux.intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>