Call ptid_get_pid instead of get_thread_id in aarch64_linux_get_debug_reg_capacity

Call ptid_get_pid instead of get_thread_id in aarch64_linux_get_debug_reg_capacity

aarch64_linux_get_debug_reg_capacity is called by
aarch64_linux_child_post_startup_inferior, and argument ptid is created in
inf-ptrace.c:inf_ptrace_create_inferior,

  /* On some targets, there must be some explicit actions taken after
     the inferior has been started up.  */
  target_post_startup_inferior (pid_to_ptid (pid));

so in aarch64_linux_get_debug_reg_capacity, we can get pid by ptid_get_pid,
and don't need to use get_thread_id.

gdb:

2015-07-17  Yao Qi  <yao.qi@linaro.org>

	* aarch64-linux-nat.c (aarch64_linux_get_debug_reg_capacity): Call
	ptid_get_pid instead of get_thread_id.