[JITLink][MachO] Add support for non-subsections-via-symbols objects.

This patch updates MachOLinkGraphBuilder to honor the MH_SUBSECTIONS_VIA_SYMBOLS
flag. Prior to this patch we assumed MH_SUBSECTIONS_VIA_SYMBOLS, but never
checked the flag.

If MH_SUBSECTIONS_VIA_SYMBOLS is set (the default for MachO output on modern
compilers) then MachOLinkGraphBuilder will break MachO section content into
jitlink::Blocks on symbol boundaries. (This is how JITLink has always handled
MachO sections previously).

If MH_SUBSECTIONS_VIA_SYMBOLS is not set then MachOLinkGraphBuilder will create
a single jitlink::Block for each MachO section.

Existing hand-written testcases that were _not_ using the
.subsections_via_symbols directive are updated to use it. A new testcase for
non-subsections-via-symbols behavior is included.