Implement "to_stopped_by_hw_breakpoint" for x86 debug registers.

Report that a thread is stopped by a hardware breakpoint if a non-data
watchpoint is set in DR6.  This change should be a no-op since a target
still needs to implement the "to_supports_stopped_by_hw_breakpoint"
method before this function is used.

gdb/ChangeLog:

	* nat/x86-dregs.c (x86_dr_stopped_by_hw_breakpoint): New function.
	* nat/x86-dregs.h (x86_dr_stopped_by_hw_breakpoint): New
	prototype.
	* x86-nat.c (x86_stopped_by_hw_breakpoint): New function.
	(x86_use_watchpoints): Set "stopped_by_hw_breakpoint" target
	method.