Enable ProcessMachCore plugin on non-apple platforms

Summary:
The plugin already builds fine on other platforms (linux, at least). All
that was necessary was to revitalize the hack in PlatformDarwinKernel
(not a very pretty hack, but it gets us going at least).

I haven't done a thorough investigation of the state of the plugin on
other platforms, but at least the two core file tests we have seem to
pass, so I enable them.

Reviewers: JDevlieghere, jasonmolenda

Subscribers: lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D47133

llvm-svn: 332997