[compiler-rt] [compiler-rt] Correctly detect cross-compiling for adding flags (PR #138868)

[Petr Hosek via llvm-commits]

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

Note that `CMAKE_CROSSCOMPILING` has a slightly different semantics, specifically it's `TRUE` whenever `CMAKE_SYSTEM_NAME` was set manually. I'd prefer using `CMAKE_CROSSCOMPILING` but it may be a potentially breaking change.

https://github.com/llvm/llvm-project/pull/138868