selftests/bpf: Prefer static linking for LLVM libraries

It is not always convenient to have LLVM libraries installed inside CI
rootfs images, thus request static libraries from llvm-config.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20240905081401.1894789-4-pulehui@huaweicloud.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>