llvm: add recipe for just the LLVM libraries

llvm: add recipe for just the LLVM libraries

Whilst it's convenient to build all of the LLVM project in one big
recipe, that's not ideal when we may just need LLVM on target and not
the rest.

Bring back a LLVM recipe that can be used by both clang (shortly) and
Rust (in the future)

Set the build type to MinSizeRel and DEBUG_LEVELFLAG to -g1 (instead of
the default, -g): the LLVM debug symbols are very large (several
gigabytes) and this reduces them to hundreds of megabytes.

Signed-off-by: Ross Burton <ross.burton@arm.com>