S390: Defer PER info update until resume

S390: Defer PER info update until resume

For multi-threaded inferiors on S390 GNU/Linux targets, GDB tried to
update the PER info via ptrace() in a newly attached thread before
assuring that the thread is stopped.  Depending on the timing, this
could lead to a GDB internal error.  The patch defers the PER info
update until just before resuming the thread.

gdb/ChangeLog:

	* s390-linux-nat.c (struct arch_lwp_info): New.
	(s390_fix_watch_points): Rename to...
	(s390_prepare_to_resume): ...this.  Skip the PER info update
	unless the watch points have changed.
	(s390_refresh_per_info, s390_new_thread): New functions.
	(s390_insert_watchpoint): Call s390_refresh_per_info instead of
	s390_fix_watch_points.
	(s390_remove_watchpoint): Likewise.
	(_initialize_s390_nat): Reflect renaming of s390_fix_watch_points.
	Register s390_prepare_to_resume.