Another place for CMAKE_HOST_UNIX

If we don't create the target, don't try to add it as a dependency.

After r265595, we were only creating the SanitizerLintCheck when
`CMAKE_HOST_UNIX` was true.

CMake was emitting a warning:
  The dependency target "SanitizerLintCheck" of target "check-ubsan" does not
  exist.

llvm-svn: 265613