Explicitly add -Wl,--export-all-symbols on mingw/cygwin.

Looks like cmake on windows is not expanding ENABLE_EXPORTS to
-Wl,--export-all-symbols on mingw or cygwin, so add this back.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178730 91177308-0d34-0410-b5e6-96231b3b80d8