lldb arm64 import.

These changes were written by Greg Clayton, Jim Ingham, Jason Molenda.

It builds cleanly against TOT llvm with xcodebuild.  I updated the
cmake files by visual inspection but did not try a build.  I haven't
built these sources on any non-Mac platforms - I don't think this
patch adds any code that requires darwin, but please let me know if
I missed something.

In debugserver, MachProcess.cpp and MachTask.cpp were renamed to
MachProcess.mm and MachTask.mm as they picked up some new Objective-C
code needed to launch processes when running on iOS.