s390/bpf: Add s390 JIT support for timed may_goto

s390/bpf: Add s390 JIT support for timed may_goto

The verifier provides an architecture-independent implementation of the
may_goto instruction, which is currently used on s390x, but it has a
downside: there is no way to prevent progs using it from running for a
very long time.

The solution to this problem is an alternative timed implementation,
which requires architecture-specific bits. Its availability is signaled
to the verifier by bpf_jit_supports_timed_may_goto() returning true.

The verifier then emits a call to arch_bpf_timed_may_goto() using a
non-standard calling convention. This function must act as a trampoline
for bpf_check_timed_may_goto().

Implement bpf_jit_supports_timed_may_goto(), account for the special
calling convention in the BPF_CALL implementation, and implement
arch_bpf_timed_may_goto().

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250821113339.292434-2-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>