Sketch out the armv7 and arm64 core file writing support in

Sketch out the armv7 and arm64 core file writing support in
ObjectFileMachO.  It's close but we seem to be missing some
of the memory region segments - not exactly sure how that's
happening.  The register context writing into the LC_THREAD
load commands is working correctly though.

Slightly reordered the arm64 definitions in ArchSpec.cpp so
when we look for an arm64 core file definiton we're getting
a cpu subtype of CPU_ANY which we can't put in the mach
header of a core file.  Make the first definition we find by
linear search have the currently correct '1' cpu subtype.

llvm-svn: 221743