[clang][PowerPC] PPC64 VAArg fix right-alignment for aggregates fit in register

PPC64 ABI pass aggregates smaller than a register into the least
significant bits of the register. In the case of variadic functions,
they will end up right-aligned in their argument slots in the argument
area on big-endian targets. Apply right-alignment for these aggregates.

Fixes #55900.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D133338