s390/bpf: Implement exceptions

s390/bpf: Implement exceptions

Implement the following three pieces required from the JIT:

- A "top-level" BPF prog (exception_boundary) must save all
  non-volatile registers, and not only the ones that it clobbers.
- A "handler" BPF prog (exception_cb) must switch stack to that of
  exception_boundary, and restore the registers that exception_boundary
  saved.
- arch_bpf_stack_walk() must unwind the stack and provide the results
  in a way that satisfies both bpf_throw() and exception_cb.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240703005047.40915-3-iii@linux.ibm.com