Add mach-o corefile support for platform binaries

Add support for recognizing a platform binary in the ObjectFileMachO
method that parses the "load binary" LC_NOTEs in a corefile.

A bit of reorganization to ProcessMachCore::DoLoadCore to separate
all of the unrelated things being done in that method into their own
separate methods, as well as small fixes to improve the handling of
a corefile with multiple kernel images in the corefile.

Differential Revision: https://reviews.llvm.org/D133680
rdar://98754861