fix arm __a_barrier_oldkuser when built as thumb

as noted in commit 05870abe, mov lr,pc
is not a valid method for saving the return address in code that might
be built as thumb.

this one is unlikely to matter, since any ISA level that has thumb2
should also have native implementations of atomics that don't involve
kuser_helper, and the affected code is only used on very old kernels
to begin with.

src/thread/arm/atomics.s

@@ -15,10 +15,10 @@ __a_barrier_oldkuser:
 	mov r1,r0
 	mov r2,sp
 	ldr ip,=0xffff0fc0
-	mov lr,pc
-	mov pc,ip
+	bl 1f
 	pop {r0,r1,r2,r3,ip,lr}
 	bx lr
+1:	bx ip
 
 .global __a_barrier_v6
 .hidden __a_barrier_v6