selftests/bpf: Add test to access int ptr argument in tracing program

Adding verifier test for accessing int pointer argument in
tracing programs.

The test program loads 2nd argument of bpf_modify_return_test
function which is int pointer and checks that verifier allows
that.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211208193245.172141-3-jolsa@kernel.org