(ghc-9.6 backport) Restore mingwex dependency on Windows, remove mingwex symbol stubs in RTS
This backports the commits from !10360 (closed) (specifically, 2b1a4abe and 28954758) to the ghc-9.6
branch. As noted in #23309 (comment 506094), this is important to prevent linker regressions in Hackage libraries that were implicitly depending on mingwex
always being present.