LLVM r600 target + libclc-git

libclc is only available off git without tagged stable versions for now, however it's required by current stable Mesa for OpenCL support. This way, we can tag it with SHA, and keep it there (or bump when necessary).
Those `provides/replaces` should be `libclc`, `libclc-devel`, `libclc-ptx` or `libclc-r600` instead.