add xaddb (byte) to nix kernel

Reference: /n/atom/patch/applied2013/xaddb
Date: Mon Dec  2 19:43:39 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/l64v.s	Mon Dec  2 19:43:28 2013
+++ /sys/src/nix/k10/l64v.s	Mon Dec  2 19:43:28 2013
@@ -337,6 +337,12 @@
 	DECL	AX
 	RET
 
+TEXT xaddb(SB), 1, $-4
+	XORL	AX, AX
+	INCL	AX
+	LOCK;	XADDB AX, (BP)
+	RET
+
 /*
  * Label consists of a stack pointer and a programme counter
  */