[PowerPC] [Clang] Fix alignment of 128-bit float types

According to ELF v2 ABI, both IEEE 128-bit and IBM extended floating
point variables should be quad-word (16 bytes) aligned. Previously, only
vector types are considered aligned as quad-word on PowerPC.

This patch will fix incorrectness of IEEE 128-bit float argument in
va_arg cases.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D91596