Improve PowerPC unwind support

Summary:
Taking advantage of the new 'CFAIsRegisterDereferenced' CFA register type, add
full stack unwind support to the PowerPC/PowerPC64 ABI.  Also, add a new
register set for powerpc32-on-64, so the register sizes are correct.  This also
requires modifying the ProcessMonitor to add support for non-uintptr_t-sized
register values.

Reviewers: jasonmolenda, emaste

Subscribers: emaste, lldb-commits

Differential Revision: http://reviews.llvm.org/D6183

llvm-svn: 221789