Restore mingwex dependency on Windows, remove mingwex symbol stubs in RTS
Restore mingwex dependency on Windows
This partially reverts some of the changes in !9475 (closed) to make base and
ghc-prim depend on the mingwex library on Windows. It also restores the
RTS's stubs for mingwex-specific symbols such as _lock_file.
This is done because the C runtime provides libmingwex nowadays, and
moreoever, not linking against mingwex requires downstream users to link
against it explicitly in difficult-to-predict circumstances. Better to always
link against mingwex and prevent users from having to do the guesswork
themselves.
See !10360 (comment 495873) for the discussion that led to this.
RtsSymbols.c: Remove mingwex symbol stubs
As of !9475 (closed), the RTS now links against ucrt instead of msvcrt on Windows,
which means that the RTS no longer needs to declare stubs for the __mingw_*
family of symbols. Let's remove these stubs to avoid confusion.
Fixes #23309.