Reland [PPC64] Port to ppc64le - initial version

Initial working version of libunwind for PowerPC 64. Tested on
little-endian ppc64 host only.
Based on the existing PowerPC 32 code.

It supports:

- context save/restore (unw_getcontext, unw_init_local, unw_resume)
- read/write from/to saved registers
- backtrace (unw_step)

Patch by Leandro Lupori!

Differential Revision: https://reviews.llvm.org/D41386

Now builds with LIBUNWIND_ENABLE_CROSS_UNWINDING=ON should
work.

llvm-svn: 321680