[PowerPC][NFC] Macro for register set defs for the Asm Parser

We have some unfortunate code in the back end that defines a bunch of register
sets for the Asm Parser. Every time another class is needed in the parser, we
have to add another one of those definitions with explicit lists of registers.
This NFC patch simply provides macros to use to condense that code a little bit.

Differential revision: https://reviews.llvm.org/D54433

llvm-svn: 350156