The default Altivec ABI was implemented but the clang error for specifying
its use still remains. Users could get around this but not specifying the
type of Altivec ABI but we need to remove the error.
Reviewed By: jsji
Differential Revision: https://reviews.llvm.org/D102094
Added support for the options mabi=vec-extabi and mabi=vec-default which are analogous to qvecnvol and qnovecnvol when using XL on AIX.
The extended Altivec ABI on AIX is enabled using mabi=vec-extabi in clang and vec-extabi in llc.
Reviewed By: Xiangling_L, DiggerLin
Differential Revision: https://reviews.llvm.org/D89684