Remove a VEC from s390-linux-nat.c

Remove a VEC from s390-linux-nat.c

This removes a use of VEC fro s390-linux-nat.c, replacing it with
std::vector.

Tested using the Fedora-s390x-m64 buildbot builder.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tromey@adacore.com>

	* s390-linux-nat.c (s390_watch_area): Remove typedef.  Don't
	declare VEC.
	(struct s390_debug_reg_state) <watch_areas, break_areas>: Now
	std::vector.
	(struct s390_process_info): Add initializers.
	(s390_add_process): Use new.
	(s390_linux_nat_target::low_forget_process): Use delete.
	(s390_linux_nat_target::low_new_fork)
	(s390_linux_nat_target::stopped_by_watchpoint)
	(s390_linux_nat_target::low_prepare_to_resume)
	(s390_linux_nat_target::insert_watchpoint)
	(s390_linux_nat_target::insert_hw_breakpoint)
	(s390_linux_nat_target::remove_watchpoint)
	(s390_linux_nat_target::remove_hw_breakpoint): Update.