s390/bpf: implement BPF_PROBE_MEM

This is a s390 port of x86 commit 3dec541b ("bpf: Add support for BTF
pointers to x86 JIT").

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>