Revert "Revert "Fix LLVMgold plugin name/path for non-Linux.""

Summary:
Relanding https://reviews.llvm.org/D35739 which was reverted because
it broke the tests on non-Linux. The tests have been fixed to be
platform agnostic, and additional tests have been added to make sure
that the plugin has the correct extension on each platform
(%pluginext doesn't work in CHECK lines).

Reviewers: srhines, pirama

Reviewed By: srhines

Subscribers: emaste, mehdi_amini, eraman, cfe-commits

Differential Revision: https://reviews.llvm.org/D36769

llvm-svn: 310960