s390/bpf: Implement bpf_jit_supports_subprog_tailcalls()

Allow mixing subprogs and tail calls by passing the current tail
call count to subprogs.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230129190501.1624747-6-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>