[cmake] Change WIN32 test to CMAKE_HOST_WIN32
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@335961 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm