add cas32 to arm for pi2

Reference: /n/atom/patch/applied/armcas32
Date: Sun Jan  3 19:57:15 CET 2016
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libc/arm/atom.s	Sun Jan  3 19:57:05 2016
+++ /sys/src/libc/arm/atom.s	Sun Jan  3 19:57:06 2016
@@ -8,6 +8,7 @@
  */
 
 TEXT	cas+0(SB),0,$0		/* r0 holds p */
+TEXT	cas32+0(SB),0,$0		/* r0 holds p */
 TEXT	casp+0(SB),0,$0		/* r0 holds p */
 	MOVW	ov+4(FP), R1
 	MOVW	nv+8(FP), R2