Rework PPC64 calls. Now we have a LR8/CTR8 register which the PPC64 calls

clobber.  This allows LR8 to be save/restored correctly as a 64-bit quantity,
instead of handling it as a 32-bit quantity.  This unbreaks ppc64 codegen when
the code is actually located above the 4G boundary.

llvm-svn: 31734