Enable building LLVMgold.dll under mingw.

Summary:
Plugins can (and should) be enabled under mingw if we are building
libLLVM.dll, so this is just a missed case. This allows LLVMgold.dll to
be built now under mingw.

Reviewers: llvm-commits, pirama, beanz, chapuni

Reviewed By: chapuni

Subscribers: chapuni, mgorny

Differential Revision: https://reviews.llvm.org/D37116

llvm-svn: 311973