Fix LLVMgold plugin name/path for non-Linux.

Summary:
It's only named LLVMgold.so on Linux. Fix the name for Windows and
Darwin.

Also fix the path for Windows so binutils doesn't have to.

Reviewers: srhines, pirama

Reviewed By: srhines

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D35739

llvm-svn: 310895