[PowerPC] Add combined ELF ABI and 32/64 bit queries to the subtarget. [NFC]

A lot of places in the code combine checks for both ABI (SVR4/Darwin/AIX) and
addressing mode (64-bit vs 32-bit). In an attempt to make some of the code more
readable I've added a couple functions that combine checking for the ELF abi and
64-bit/32-bit code at once. As we add more AIX support I intend to add similar
functions for the AIX ABI.

Differential Revision: https://reviews.llvm.org/D65814

llvm-svn: 369658