[PowerPC][AIX] Add support for lowering int/float/double formal arguments.

This patch adds LowerFormalArguments_AIX, support is added for lowering
int, float, and double formal arguments into general purpose and
floating point registers only.

The aix calling convention testcase have been redone to test for caller
and callee functionality in the same lit test.

Patch by Zarko Todorovski!

Differential Revision: https://reviews.llvm.org/D69578