selftests/bpf: Add a sign-extension test for kfuncs

s390x ABI requires the caller to zero- or sign-extend the arguments.
eBPF already deals with zero-extension (by definition of its ABI), but
not with sign-extension.

Add a test to cover that potentially problematic area.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-15-iii@linux.ibm.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>