[llvm] Set Support system_libs if WIN32, not just MSVC or MINGW (PR #95505)

[Martin Storsjö via llvm-commits]

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

LGTM, thanks!

Note that the PR description (the first post) is what ends up as the actual commit message, when merged with "squash and merge" (although it's possible to edit it when merging). So it can be worthwhile to polish it up and split out parts that aren't intended as the final commit message (e.g. the references to me) ;-)

https://github.com/llvm/llvm-project/pull/95505