lib/raid6: Build proper raid6test files on powerpc

Previously the raid6 test Makefile did not build the POWER specific files
(altivec and vpermxor).
This patch fixes the bug, so that all appropriate files for powerpc are built.

This patch also fixes the missing and mismatched ifdef statements to allow the
altivec.uc file to be built correctly.

Signed-off-by: Matt Brown <matthew.brown.dev@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>