[PowerPC][AIX] Add support for varargs for complex types on AIX

Remove the previous error and add support for special handling of small
complex types as in PPC64 ELF ABI. As in, generate code to load from
varargs location and pack it in a temp variable, then return a pointer to
the struct.

Reviewed By: sfertile

Differential Revision: https://reviews.llvm.org/D106393