s390/bpf: Use lg(f)rl when long displacement cannot be used

If literal pool grows past 524287 mark, it's no longer possible to use
long displacement to reference literal pool entries. In JIT setting
maintaining multiple literal pool registers is next to impossible, since
we operate on one instruction at a time.

Therefore, fall back to loading literal pool entry using PC-relative
addressing, and then using a register-register form of the following
machine instruction.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191118180340.68373-6-iii@linux.ibm.com