* s390-tdep.c: Don't use a call dummy.

* s390-tdep.c: Don't use a call dummy.
(s390_fix_call_dummy, s390_pc_in_call_dummy): Delete.
(s390_push_return_address): Put the address of the inferior call
breakpoint in r14.
(s390_gdbarch_init):
- Provide trivial definition of s390_call_dummy_words; register it
  with the gdbarch appropriately.
- Delete S390x_call_dummy_words.
- Gather inferior-call-related settings into a group.
- Use generic dummy frames.
- Put the inferior call breakpoint at the entry point.
- Use generic gdbarch methods: pc_in_call_dummy_at_entry_point,
  generic_push_dummy_frame, generic_fix_call_dummy.
- There is a call dummy breakpoint offset; it's zero.