so that va_start/va_arg/et.al. will walk arguments correctly for Cell SPU.
N.B.: Because neither clang nor llvm-gcc-4.2 can be built for CellSPU, this is
still unexorcised code.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58415 91177308-0d34-0410-b5e6-96231b3b80d8