gallivm: Detect VSX separately from Altivec

gallivm: Detect VSX separately from Altivec

Previously gallivm would attempt to use VSX instructions on all systems
where it detected that Altivec is supported; however, VSX was added to
POWER long after Altivec, causing lots of crashes on older POWER/PPC
hardware, e.g. PPC Macs. By detecting VSX separately from Altivec we can
automatically disable it on hardware that supports Altivec but not VSX

Signed-off-by: Vicki Pfau <vi@endrift.com>