selftests/bpf: Support dynamically linking LLVM if static is not available

selftests/bpf: Support dynamically linking LLVM if static is not available

Since 67ab80a01886 ("selftests/bpf: Prefer static linking for LLVM
libraries"), only statically linking test_progs is supported. However,
some distros only provide a dynamically linkable LLVM.

This commit adds a fallback for dynamically linking LLVM if static
linking is not available. If both options are available, static linking
is chosen.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/872b64e93de9a6cd6a7a10e6a5c5e7893704f743.1738276344.git.dxu@dxuuu.xyz