[PowerPC] Emit error for Altivec vector initializations when -faltivec-src-compat=gcc is specified

Under the -faltivec-src-compat=gcc option, AltiVec vector initialization should
be treated as if they were compiled with gcc - which is, to emit an error when
the vectors are initialized in the parenthesized or non-parenthesized manner.
This patch implements this behaviour.

Differential Revision: https://reviews.llvm.org/D106410