cinap's rex fix

Reference: /n/atom/patch/applied/8dbrex
Date: Tue Apr 15 15:52:28 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libmach/8db.c	Tue Apr 15 15:52:23 2014
+++ /sys/src/libmach/8db.c	Tue Apr 15 15:52:25 2014
@@ -1888,7 +1888,7 @@
 {
 	if (ip->mod == 3) {
 		if (ip->osize == 'B')
-			bprint(ip, (ip->rex & REXB? breg64: breg)[ip->base]);
+			bprint(ip, (ip->rex? breg64: breg)[ip->rex&REXB? ip->base+8: ip->base]);
 		else if(ip->rex & REXB)
 			bprint(ip, "%s%s", ANAME(ip), reg[ip->base+8]);
 		else