[llvm] [LLVM][TargetParser] Handle -msys targets the same as -cygwin. (PR #136817)

[Martin Storsjö via llvm-commits]

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

This looks reasonable to me, and doesn't seem controversial.

I'll leave it open for 24h after approving, in case someone else (across all timezones) want to chime in still - I'll try to merge it after that if I remember (or @mati865 also should be able to).

https://github.com/llvm/llvm-project/pull/136817