[AIX][PowerPC] Remove error when specifying mabi=vec-default on AIX

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