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@xxxxxxxxx>