fix arm clone syscall bug (no effect unless app uses clone)

the code to exit the new thread/process after the start function
returns was mixed up in its syscall convention.

src/thread/arm/clone.s

@@ -27,8 +27,7 @@ clone:
 	bne 1f
 	mov lr,pc
 	mov pc,r5
-2:	mov r1,r0
-	mov r0,#1
+2:	mov r7,#1
 	svc 0
 
 1:	mov lr,pc