s390/bpf: Use relative long branches

Currently maximum JITed code size is limited to 64k, because JIT can
emit only relative short branches, whose range is limited by 64k in both
directions.

Teach JIT to use relative long branches. There are no compare+branch
relative long instructions, so using relative long branches consumes
more space due to having to having to emit an explicit comparison
instruction. Therefore do this only when relative short branch is not
enough.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191118180340.68373-2-iii@linux.ibm.com