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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> This is _actually_ used on Windows, so removing `UNUSED`

No, it's not used in i386 mode. It's used in some modes, but not all - therefore it is marked as unused to silence warnings in those build configuration. See 979c38cc74f4e96c5e1bee1f966a81038fff1ed6 which introduced the attribute here.

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