[compiler-rt] Fixing UB on Windows for trampoline allocations (PR #85639)

[Martin Storsjö via llvm-commits]

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

LGTM, thanks!

(At this point, it'd be good to update the commit description, which ends up as the commit message, to not refer to removing `UNUSED` any longer. Or whoever does the merge can do that fixup of the commit message before finishing the merge.)

https://github.com/llvm/llvm-project/pull/85639