S390: Migrate watch areas from list to VEC type

S390: Migrate watch areas from list to VEC type

For S390, the list of active watchpoints is maintained in a list based
at "watch_base".  This refactors the list to a vector "watch_areas".

gdb/ChangeLog:

	* s390-linux-nat.c (s390_watch_area): New typedef.  Define a VEC.
	(watch_base): Remove variable.
	(watch_areas): New variable.
	(s390_stopped_by_watchpoint): Transform operations on the
	watch_base list to equivalent operations on the watch_areas VEC.
	(s390_prepare_to_resume): Likewise.
	(s390_insert_watchpoint): Likewise.
	(s390_remove_watchpoint): Likewise.