Move low-level Linux x86 debug register code to a shared file

Move low-level Linux x86 debug register code to a shared file

This commit moves the now-identical low-level Linux x86 debug register
code from gdb/x86-linux-nat.c and gdb/gdbserver/linux-x86-low.c into a
new shared file gdb/nat/x86-linux-dregs.c.

gdb/ChangeLog:

	* nat/x86-linux-dregs.h: New file.
	* nat/x86-linux-dregs.c: Likewise.
	* Makefile.in (HFILES_NO_SRCDIR): Add nat/x86-linux-dregs.h.
	(x86-linux-dregs.o): New rule.
	* config/i386/linux.mh (NATDEPFILES): Add x86-linux-dregs.o.
	* config/i386/linux64.mh (NATDEPFILES): Likewise.
	* x86-linux-nat.c: Include nat/x86-linux-dregs.h.
	(u_debugreg_offset): Moved to nat/x86-linux-dregs.c.
	(x86_linux_dr_get): Likewise.
	(x86_linux_dr_set): Likewise.
	(x86_linux_dr_get_addr): Likewise.
	(x86_linux_dr_get_control): Likewise.
	(x86_linux_dr_get_status): Likewise.
	(update_debug_registers_callback): Likewise.
	(x86_linux_dr_set_control): Likewise.
	(x86_linux_dr_set_addr): Likewise.
	(x86_linux_update_debug_registers): Likewise.

gdb/gdbserver/ChangeLog:

	* Makefile.in (x86-linux-dregs.o): New rule.
	* configure.srv: Add x86-linux-dregs.o to relevant targets.
	* linux-x86-low.c: Include nat/x86-linux-dregs.h.
	(u_debugreg_offset): Moved to nat/x86-linux-dregs.c.
	(x86_linux_dr_get): Likewise.
	(x86_linux_dr_set): Likewise.
	(update_debug_registers_callback): Likewise.
	(x86_linux_dr_set_addr): Likewise.
	(x86_linux_dr_get_addr): Likewise.
	(x86_linux_dr_set_control): Likewise.
	(x86_linux_dr_get_control): Likewise.
	(x86_linux_dr_get_status): Likewise.
	(x86_linux_update_debug_registers): Likewise.