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