[MinGW] Don't mark external variables as DSO local

Since MinGW supports automatically importing external variables from
DLLs even without the DLLImport attribute, we shouldn't mark them
as DSO local unless we actually know them to be local for sure.

Keep marking thread local variables as DSO local.

Differential Revision: https://reviews.llvm.org/D51382

llvm-svn: 340941