Enable support for x86 debug registers on NetBSD.

Enable support for x86 debug registers on NetBSD.

NetBSD recently added PT_GETDBREGS and PT_SETDBREGS ptrace operations
that match the existing ones supported by x86-bsd-nat.c.  NetBSD's
headers do not provide the DBREG_DRX helper macro, so define a local
version in x86-bsd-nat.c.  In addition, add the x86-nat.o and x86-dregs.o
object files to the native NetBSD x86 build targets.

gdb/ChangeLog:

	* configure.nat: Add "x86-nat.o x86-dregs.o" for NetBSD/amd64 and
	NetBSD/i386.
	* x86-bsd-nat.c [!DBREG_DRX && __NetBSD__]: Define DBREG_DRX.