[cmake] Change WIN32 test to CMAKE_HOST_WIN32

The test is about what can be run on the host, not the cmake target.
When cross-compiling (compiler-rt at least) on Windows, we end up with
lit being unable to run llvm-lit because it can't find the llvm-lit
module.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335961 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed