[compiler-rt] Correctly detect cross-compiling for adding flags

[aaronpuchert]

This started with "Android is always cross-compiled" in commit aa9d74c, and continued with individual architectures like ARM in 6759fd9. Later Apple was carved out in 8887c63. I think we should just use the variable provided by CMake instead.

[petrhosek]

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.