[PowerPC][AIX] Implement by-val caller arguments in a single register.

This is the first of a series of patches that adds caller support for
by-value arguments. This patch add support for arguments that are passed in a
single GPR.

There are 3 limitation cases:
-The by-value argument is larger than a single register.
-There are no remaining GPRs even though the by-value argument would
otherwise fit in a single GPR.
-The by-value argument requires alignment greater than register width.

Future patches will be required to add support for these cases as well
as for the callee handling (in LowerFormalArguments_AIX) that
corresponds to this work.

Differential Revision: https://reviews.llvm.org/D75863