lldb fix ARM64 register access

Apparently, PEEKUSER/POKEUSER is something x86 specific, so I had to rework it for AArch64. This fixes assertion that occurs whenever lldb started on AArch64 device tried to read PC register (or any other register)

See http://reviews.llvm.org/D5232 for more details.

Change by Paul Osmialowski.

llvm-svn: 217691