Support mingw-w64 and mingw.org toolchains at any install location.

No more hardcoded paths: clang will use -sysroot as gcc root location if
provided. Otherwise, it will search for gcc on the path. If not found it
will use the driver installed location.

 http://reviews.llvm.org/D5268

Patch by Ruben Van Boxem, Martell Malone, Yaron Keren.
Reviewed by Reid Kleckner.

llvm-svn: 241241