Properly handle the mftb instruction.

The mftb instruction was incorrectly marked as deprecated in the PPC
Backend. Instead, it should not be treated as deprecated, but rather be
implemented using the mfspr instruction. A similar patch was put into GCC last
year. Details can be found at:

https://sourceware.org/ml/binutils/2014-11/msg00383.html.
This change will replace instances of the mftb instruction with the mfspr
instruction for all CPUs except 601 and pwr3. This will also be the default
behaviour.

Additional details can be found in:

https://llvm.org/bugs/show_bug.cgi?id=23680

Phabricator review: http://reviews.llvm.org/D10419

llvm-svn: 239827