clang: split lld into a separate lld recipe

clang: split lld into a separate lld recipe

The lld linker is a standalone project under the LLVM umbrella that just
depends on libLLVM and nothing else, such as clang.

To reduce the build time of clang if lld is not being used, split it out
into a separate recipe.

To ensure that lld is present if needed, the clang-cross recipe will
depend on lld-native if ld-is-lld is enable.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>