[PowerPC] ABI support for the QPX vector instruction set

Support for the QPX vector instruction set, used on the IBM BG/Q supercomputer,
has recently been added to the LLVM PowerPC backend. This vector instruction
set requires some ABI modifications because the ABI on the BG/Q expects
<4 x double> vectors to be provided with 32-byte stack alignment, and to be
handled as native vector types (similar to how Altivec vectors are handled on
mainline PPC systems). I've named this ABI variant elfv1-qpx, have made this
the default ABI when QPX is supported, and have updated the ABI handling code
to provide QPX vectors with the correct stack alignment and associated
register-assignment logic.

llvm-svn: 231960