Log Message:
Prepare gdb/amd64bsd-nat.c for Debug Registers (watchpoints) on NetBSD

Define DBREG_DRX when it's undefined to:
    #define        DBREG_DRX(d,x)  ((d)->dr[(x)])

This has been copied from FreeBSD:
/usr/include/x86/reg.h:#define	DBREG_DRX(d,x)	((d)->dr[(x)])

No functional change for existing operating systems.

Sponsored by <The NetBSD Foundation>