Fix ia64-linux-nat.c

Fix ia64-linux-nat.c

PR build/23814 points out that ia64-linux-nat.c will not compile any
more.  This patch fixes the problem.  Thanks to Andreas Schwab for
trying the patch.

gdb/ChangeLog
2018-11-18  Tom Tromey  <tom@tromey.com>

	PR build/23814:
	* target-delegates.c: Rebuild.
	* ia64-linux-nat.c (class ia64_linux_nat_target)
	<have_steppable_watchpoint>: Use override.  Return true, not 1.
	(ia64_linux_nat_target::can_use_hw_breakpoint): Rename.  Remove
	"self" argument.
	(ia64_linux_nat_target::low_new_thread): Rename.
	(class ia64_linux_nat_target) <read_description>: Don't declare.
	* target.h (struct target_ops) <have_steppable_watchpoint>: Return
	bool.