mono: add LLVM support (3.4svn build)

This adds LLVM support in Mono using their custom fork, based on LLVM
3.4svn upstream. Somehow, it's gone for a while without a patch to fix
the CMakeLists.txt in the fork...

The upstream commit is based on the mono3 branch.

Signed-off-by: Austin Seipp <aseipp@pobox.com>