x86 debug address register clarifications

x86 debug address register clarifications

The loop macro ALL_DEBUG_REGISTERS does not iterate over the status or
control registers, so its name is misleading.  This commit renames it
as ALL_DEBUG_ADDRESS_REGISTERS and updates all uses.  This commit also
updates its loop conditions to an equivalent but better form, and
makes two functions use it that had previously hardwired the loop.
A comment on a related field in the x86_debug_reg_state structure is
also updated to reflect that the field refers specifically to address
registers only.

gdb/ChangeLog:

	* nat/x86-dregs.h (ALL_DEBUG_REGISTERS): Renamed as...
	(ALL_DEBUG_ADDRESS_REGISTERS): New macro.  All uses updated.
	Loop conditions	changed to equivalent form.
	(struct x86_debug_reg_state): Updated dr_ref_count comment.
	* x86-linux-nat.c (x86_linux_prepare_to_resume): Use
	ALL_DEBUG_ADDRESS_REGISTERS.

gdb/gdbserver/ChangeLog:

	* linux-x86-low.c (x86_linux_prepare_to_resume): Use
	ALL_DEBUG_ADDRESS_REGISTERS.