Document the subtarget features better, make sure that 64-bit mode, 64-bit

support, and 64-bit register use are all consistent with each other.

Add a new "IsPPC" feature, to distinguish ppc32 vs ppc64 targets, use this
to configure TargetData differently.  This not makes ppc64 blow up on lots
of stuff :)

llvm-svn: 28825