[PowerPC] Implement 64-bit ELFv2 Calling Convention in TableGen (for integers/floats...

[PowerPC] Implement 64-bit ELFv2 Calling Convention in TableGen (for integers/floats/vectors in registers)

This patch partially implements the parameter passing rules outlined in the
ELFv2 ABI within TableGen. Specifically, it implements the parameter assignment
of integers, floats, and vectors within registers - where the GPR numbering will
be "skipped" depending on the ordering of floats and vectors that appear within
a parameter list.

As we begin to adopt GlobalISel to the PowerPC backend, there is a need for a
TableGen definition that encapsulates the ELFv2 parameter passing rules. Thus,
this patch also changes the default calling convention that is returned within
the ccAssignFnForCall() function used in our GlobalISel implementation, and also
adds some additional testing of the calling convention that is implemented.

Future patches that build on top of this initial TableGen definition will aim to
add more of the ABI complexities, including support for additional types and
also in-memory arguments.

Differential Revision: https://reviews.llvm.org/D137504