fix rdrandbuf for (n&7)!=0.  since DESKEYLEN=7, this can
cause factotum issues generating keys.

Reference: /n/atom/patch/applied2013/rdrandbotch
Date: Mon Jul 15 21:22:44 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/l64v.s	Mon Jul 15 21:21:45 2013
+++ /sys/src/nix/k10/l64v.s	Mon Jul 15 21:21:45 2013
@@ -431,7 +431,7 @@
 	JMP	eights
 
 f1:
-	MOVL	RARG, CX
+	MOVLQZX	cnt+8(FP), CX
 	ANDL	$7, CX
 	SHRQ	$2, CX
 fours:
@@ -444,7 +444,7 @@
 	JMP	fours
 
 f2:
-	MOVL	RARG, CX
+	MOVLQZX	cnt+8(FP), CX
 	ANDL	$3, CX
 ones:
 	CMPL	CX, $0