Use GET_CURRENT_FRAME() to calculate the memory layout for power. This works
for both PPC64 Big and Little endian modes, so also eliminates the need for
the BIG_ENDIAN/LITTLE_ENDIAN #ifdeffery.

By trial and error, it also looks like the kPPC64_ShadowOffset64 value is
valid using (1ULL << 41) for both BE and LE, so that #if/#elif/#endif block
has also been simplified.

Differential Revision: http://reviews.llvm.org/D6044