bpf: add couple of test cases for signed extended imms

Add a couple of test cases for interpreter and JIT that are
related to an issue we faced some time ago in Cilium [1],
which is fixed in LLVM with commit e53750e1e086 ("bpf: fix
bug on silently truncating 64-bit immediate").

Test cases were run-time checking kernel to behave as intended
which should also provide some guidance for current or new
JITs in case they should trip over this. Added for cBPF and
eBPF.

  [1] https://github.com/cilium/cilium/pull/2162



Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>