bpf: add various test cases to verifier selftests

Add several test cases around ldimm64, fp arithmetic and direct
packet access.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>