s390/bpf: Implement BPF_MOV | BPF_X with sign-extension

Implement the cpuv4 register-to-register move with sign extension. It
is distinguished from the normal moves by non-zero values in
insn->off, which determine the source size. s390x has instructions to
deal with all of them: lbr, lhr, lgbr, lghr and lgfr.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230919101336.2223655-5-iii@linux.ibm.com


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