steppable, not both. The S/390 has continuable watchpoints, so delete the #definition of HAVE_STEPPABLE_WATCHPOINT.
(lin_lwp_attach_lwp, ATTACH_LWP, lin_thread_get_thread_signals, GET_THREAD_SIGNAL): Remove.
Contributed by D.J. Barrow <djbarrow@de.ibm.com> of IBM. * s390-nat.c, s390-tdep.c: New file. * config/s390/nm-linux.h, config/s390/s390.mh: New file. * config/s390/s390.mt, config/s390/s390x.mt: New file. * config/s390/tm-linux.h, config/s390/tm-s390.h: New file. * config/s390/xm-linux.h: New file. * NEWS: Update. * MAINTAINERS: Update.