Add various test_verifier selftests, and a simple xdp/tc functional
test that is being attached to veths. Also let new versions of clang
use the recently added -mcpu=probe support [1] for the BPF target,
so that it can probe the underlying kernel for BPF insn set extensions.
We could also just set this options always, where older versions just
ignore it and give a note to the user that the -mcpu value is not
supported, but given emitting the note cannot be turned off from clang
side lets not confuse users running selftests with it, thus fallback
to the default generic one when we see that clang doesn't support it.
Also allow CPU option to be overridden in the Makefile from command
line.

  [1] https://github.com/llvm-mirror/llvm/commit/d7276a40d87b89aed89978dec6457a5b8b3a0db5Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>