selftests/bpf: Test ARG_PTR_TO_LONG arg type

Test that verifier handles new argument types properly, including
uninitialized or partially initialized value, misaligned stack access,
etc.

Example of output:
  #456/p ARG_PTR_TO_LONG uninitialized OK
  #457/p ARG_PTR_TO_LONG half-uninitialized OK
  #458/p ARG_PTR_TO_LONG misaligned OK
  #459/p ARG_PTR_TO_LONG size < sizeof(long) OK
  #460/p ARG_PTR_TO_LONG initialized OK

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>