s390/bpf: Implement unconditional byte swap

Implement the cpuv4 unconditional byte swap, which is encoded as
BPF_ALU64 | BPF_END | BPF_FROM_LE. Since s390x is big-endian, it's
the same as the existing BPF_ALU | BPF_END | BPF_FROM_LE.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230919101336.2223655-7-iii@linux.ibm.com


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