This patch adds Altivec support for RAID-6, if appropriately configured 
on the ppc or ppc64 architectures.  Note that it changes the compile 
flags for ppc64 in order to handle -maltivec correctly; this change was 
vetted on the ppc64 mailing list and OK'd by paulus.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>