radeon/llvm: Remove backend code from Mesa

radeon/llvm: Remove backend code from Mesa

This code now lives in an external tree.

For the next Mesa release fetch the code from the master branch
of this LLVM repo:
http://cgit.freedesktop.org/~tstellar/llvm/

For all subsequent Mesa releases, fetch the code from the official LLVM
project:
www.llvm.org