Introduce current_lwp_ptid

Introduce current_lwp_ptid

This commit introduces a new function, current_lwp_ptid, that
shared Linux code can use to obtain the ptid of the current
lightweight process.

gdb/ChangeLog:

	* nat/linux-nat.h (current_lwp_ptid): New declaration.
	* linux-nat.c (current_lwp_ptid): New function.
	* x86-linux-nat.c: Include nat/linux-nat.h.
	(x86_linux_dr_get_addr): Use current_lwp_ptid.
	(x86_linux_dr_get_control): Likewise.
	(x86_linux_dr_get_status): Likewise.
	(x86_linux_dr_set_control): Likewise.
	(x86_linux_dr_set_addr): Likewise.

gdb/gdbserver/ChangeLog:

	* linux-low.c (current_lwp_ptid): New function.
	* linux-x86-low.c: Include nat/linux-nat.h.
	(x86_dr_low_get_addr): Use current_lwp_ptid.
	(x86_dr_low_get_control): Likewise.
	(x86_dr_low_get_status): Likewise.