[PPC64LE] Run some existing Altivec tests on powerpc64le as well

There are several Altivec tests that formerly ran only on big-endian
targets (and in some cases only on 32-bit targets).  It is useful to
verify these on little-endian targets as well.

While testing these, I discovered a typo in <altivec.h>.  This is also
fixed by this patch.

llvm-svn: 210928