[clang] [Driver] Add winsysroot alias to the GNU driver (PR #95320)

[via cfe-commits]

https://github.com/zmodem approved this pull request.

lgtm

A test would be nice, but I see the other flags added in f1440bf6fd22ca0a5fc3594000e966201989fd48 don't have tests either, so maybe it's okay.

https://github.com/llvm/llvm-project/pull/95320