[gdbserver] Use iterate_over_lwps in aarch64_notify_debug_reg_change

[gdbserver] Use iterate_over_lwps in aarch64_notify_debug_reg_change

This patch makes more bits on aarch64 watchpoint between GDB and GDBserver
look similar.

gdb/gdbserver:

2015-08-25  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-low.c (aarch64_dr_update_callback_param) <pid>:
	Remove.
	(debug_reg_change_callback): Remove argument entry and add argument
	lwp.  Remove local variable thread.  Don't print thread id in the
	debugging output.  Don't check whether pid of thread equals to pid.
	(aarch64_notify_debug_reg_change): Don't set param.pid.  Call
	iterate_over_lwps instead find_inferior.