Add PPC option for full register names in asm

On non-Darwin PPC systems, we currently strip off the register name prefix
prior to instruction printing. So instead of something like this:

  mr r3, r4

we print this:

  mr 3, 4

The first form is the default on Darwin, and is understood by binutils, but not
yet understood by our integrated assembler. Once our integrated-as understands
full register names as well, this temporary option will be replaced by tying
this functionality to the verbose-asm option. The numeric-only form is
compatible with legacy assemblers and tools, and is also gcc's default on most
PPC systems. On the other hand, it is harder to read, and there are some
analysis tools that expect full register names.