powerpc: Add VMX optimised xor for RAID5

Add a VMX optimised xor, used primarily for RAID5. On a POWER7 blade
this is a decent win:

   32regs    : 17932.800 MB/sec
   altivec   : 19724.800 MB/sec

The bigger gain is when the same test is run in SMT4 mode, as it
would if there was a lot of work going on:

   8regs     :  8377.600 MB/sec
   altivec   : 15801.600 MB/sec

I tested this against an array created without the patch, and also
verified it worked as expected on a little endian kernel.

[ Fix !CONFIG_ALTIVEC build -- BenH ]

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/lib/Makefile

@@ -39,3 +39,6 @@ obj-$(CONFIG_PPC_LIB_RHEAP) += rheap.o
 obj-y			+= code-patching.o
 obj-y			+= feature-fixups.o
 obj-$(CONFIG_FTR_FIXUP_SELFTEST) += feature-fixups-test.o
+
+obj-$(CONFIG_ALTIVEC)	+= xor_vmx.o
+CFLAGS_xor_vmx.o += -maltivec -mabi=altivec