[PowerPC][FP128] Fix the incorrect calling convention for IEEE long double on Power8

For now, we are using the GPR to pass the arguments/return value for fp128 on Power8,
which is incorrect. It should be VSR. The reason why we do it this way is that,
we are setting the fp128 as illegal which make LLVM try to emulate it with i128 on
Power8. So, we need to correct it as legal.

Reviewed By: Nemanjai

Differential Revision: https://reviews.llvm.org/D91527