libclc: split out of clang

libclc: split out of clang

Split the libclc subproject out of the clang recipe and into a dedicated
libclc recipe.

This is useful because libclc is the OpenCL runtime library and as such
isn't target-specific and needs a native clang to build, not a target
libllvm.

Verified that nothing is dropped by adding clang and libclc to an image
and verifying that the file list is the same before and after this
change.

We need to patch the libclc CMakeLists to allow it to use an out-of-tree
prepare_builtins binary, discussion is ongoing with upstream to resolve
this properly.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>