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.