[compiler-rt] Fix check for cross-compiling

When checking if we are cross-compiling, use `CMAKE_HOST_SYSTEM_NAME`
rather than `CMAKE_HOST_SYSTEM` which seems to have the full version
number attached.

Reviewed By: smeenai

Differential Revision: https://reviews.llvm.org/D132130

GitOrigin-RevId: ee80903a2ceb03f4617550eba55881f7b80ebb8e
1 file changed