Add llvm-rs-cc to the tools buildable under Windows.

To be able to "make llvm-rs-cc" as a Windows SDK tool, we
first need to let the build system know which folders can be
built.

Once this is done, building can be done using the mingw32
package to do cross-compiling under Linux:
  $ USE_MINGW=1 make llvm-rs-cc

Change-Id: I5538f4c52c74dad18dd50b59c9bdbcd6de3fd191