bpf, testing: Add various tail call test cases

Add several BPF kselftest cases for tail calls which test the various
patch directions, and that multiple locations are patched in same and
different programs.

  # ./test_progs -n 45
   #45/1 tailcall_1:OK
   #45/2 tailcall_2:OK
   #45/3 tailcall_3:OK
   #45/4 tailcall_4:OK
   #45/5 tailcall_5:OK
   #45 tailcalls:OK
  Summary: 1/5 PASSED, 0 SKIPPED, 0 FAILED

I've also verified the JITed dump after each of the rewrite cases that
it matches expectations.

Also regular test_verifier suite passes fine which contains further tail
call tests:

  # ./test_verifier
  [...]
  Summary: 1563 PASSED, 0 SKIPPED, 0 FAILED

Checked under JIT, interpreter and JIT + hardening.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/3d6cbecbeb171117dccfe153306e479798fb608d.1574452833.git.daniel@iogearbox.net