Remove get_thread_id

Remove get_thread_id

This patch removes get_thread_id from aarch64-linux-nat.c,
arm-linux-nat.c and xtensa-linux-nat.c.

get_thread_id was added in this commit below in 2000,

  41c49b06c471443d3baf2eaa2463a315f9b5edca
  https://sourceware.org/ml/gdb-patches/2000-04/msg00398.html

which predates the ptid_t stuff added into GDB.  Nowadays, lwpid of
inferior_ptid is only zero when the inferior is created (in
fork-child.c:fork_inferior) and its lwpid will be set after
linux_nat_wait_1 gets the first event.  After that, lwpid of
inferior_ptid is not zero for linux-nat target, then we can use
ptid_get_lwp, so this function isn't needed anymore.

Even when GDB attaches to a process, the lwp of inferior_ptid
isn't zero,  see linux-nat.c:linux_nat_attach,

  /* The ptrace base target adds the main thread with (pid,0,0)
     format.  Decorate it with lwp info.  */
  ptid = ptid_build (ptid_get_pid (inferior_ptid),
		     ptid_get_pid (inferior_ptid),
		     0);

Note that linux_nat_xfer_partial shifts lwpid to pid for inferior_ptid
temperately for calling linux_ops->to_xfer_partial, but all the
affected functions in this patch are not called in
linux_ops->to_xfer_partial.

I think we can safely remove get_thread_id for all linux native targets.

Regression tested on arm-linux and aarch64-linux.  Unable to build
native GDB and test it on xtensa-linux.

gdb:

2015-08-05  Yao Qi  <yao.qi@linaro.org>

	* aarch64-linux-nat.c (get_thread_id): Remove.
	(debug_reg_change_callback): Call ptid_get_lwp instead of
	get_thread_id.
	(fetch_gregs_from_thread): Likewise.
	(store_gregs_to_thread): Likewise.
	(fetch_fpregs_from_thread): Likewise.
	(store_fpregs_to_thread): Likewise.
	(aarch64_linux_get_debug_reg_capacity): Likewise.
	* arm-linux-nat.c (get_thread_id): Remove.
	(GET_THREAD_ID): Update macro to use ptid_get_lwp.
	* xtensa-linux-nat.c (get_thread_id): Remove.
	(GET_THREAD_ID): Update macro to use ptid_get_lwp.
	* arm-linux-nat.c (get_thread_id): Remove.
	(GET_THREAD_ID): Remove.
	(fetch_fpregs): Call ptid_get_lwp instead of GET_THREAD_ID.
	(store_fpregs, fetch_regs, store_regs): Likewise.
	(fetch_wmmx_regs, store_wmmx_regs): Likewise.
	(fetch_vfp_regs, store_vfp_regs): Likewise.
	(arm_linux_read_description): Likewise.
	(arm_linux_get_hwbp_cap): Likewise.
	* xtensa-linux-nat.c (get_thread_id): Remove.
	(GET_THREAD_ID): Remove.
	(fetch_gregs, store_gregs): Call ptid_get_lwp instead of
	GET_THREAD_ID.