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