[PPC64LE] Temporarily disable VSX support in little-endian mode

This is a preliminary patch for the PowerPC64LE support.  In stage 1
of the vector support, we will support the VMX (Altivec) instruction
set, but will not yet support the VSX instructions.  This is merely a
staging issue to provide functional vector support as soon as
possible.

llvm-svn: 210271