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).
Using the SHA1 as version component is not a good idea because xbps cannot know when it's greatest than other.

Please use a date version, i.e `version=20140721` and then checkout the latest revision of that date.