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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311973 91177308-0d34-0410-b5e6-96231b3b80d8