Vectorize gdbserver x86 debug register accessors

This commit makes gdbserver access the x86 debug register accessor
functions via the same function vector as GDB proper.  This removes
a chunk of conditional code that was previously in i386-{nat,low}.h
and leaves a single macro as the only GDB/gdbserver difference in
nat/i386-dregs.c.

gdb/
2014-06-19  Gary Benson  <gbenson@redhat.com>

	* i386-nat.h (debug_hw_points): Moved to nat/i386-dregs.c.
	(i386_dr_low_type): Moved to nat/i386-dregs.h.
	(i386_dr_low): Likewise.
	(i386_dr_low_can_set_addr): Moved to nat/i386-dregs.c.
	(i386_dr_low_set_addr): Likewise.
	(i386_dr_low_get_addr): Likewise.
	(i386_dr_low_can_set_control): Likewise.
	(i386_dr_low_set_control): Likewise.
	(i386_dr_low_get_control): Likewise.
	(i386_dr_low_get_status): Likewise.
	(i386_get_debug_register_length): Likewise.
	* nat/i386-dregs.h (i386_dr_low_type): Moved from i386-nat.h.
	(i386_dr_low): Likewise.
	* nat/i386-dregs.c (i386-low.h): Remove include.
	(i386-nat.h): Likewise.
	(nat/i386-dregs.h): New include.
	(i386_dr_low_can_set_addr): Moved from i386-nat.h.
	(i386_dr_low_set_addr): Likewise.
	(i386_dr_low_get_addr): Likewise.
	(i386_dr_low_can_set_control): Likewise.
	(i386_dr_low_set_control): Likewise.
	(i386_dr_low_get_control): Likewise.
	(i386_dr_low_get_status): Likewise.
	(i386_get_debug_register_length): Likewise.
	(debug_hw_points): Likewise.

gdb/gdbserver/
2014-06-19  Gary Benson  <gbenson@redhat.com>

	* i386-low.h (i386_dr_low_can_set_addr): Removed.
	(i386_dr_low_set_addr): Likewise.
	(i386_dr_low_get_addr): Likewise.
	(i386_dr_low_can_set_control): Likewise.
	(i386_dr_low_set_control): Likewise.
	(i386_dr_low_get_control): Likewise.
	(i386_dr_low_get_status): Likewise.
	(i386_get_debug_register_length): Likewise.
	* linux-x86-low.c (i386_dr_low_set_addr):
	Changed signature.  Made static.
	(i386_dr_low_get_addr): Likewise.
	(i386_dr_low_set_control): Likewise.
	(i386_dr_low_get_control): Likewise.
	(i386_dr_low_get_status): Likewise.
	(i386_dr_low): New global variable.
	* win32-i386-low.c (i386_dr_low_set_addr):
	Changed signature.  Made static.
	(i386_dr_low_get_addr): Likewise.
	(i386_dr_low_set_control): Likewise.
	(i386_dr_low_get_control): Likewise.
	(i386_dr_low_get_status): Likewise.
	(i386_dr_low): New global variable.