runtime: fix systemstack tracebacks on nacl/arm

For #11956.

Change-Id: Ic9b57cafa197953cc7f435941e44d42b60b3ddf0
Reviewed-on: https://go-review.googlesource.com/13011Reviewed-by: Dave Cheney <dave@cheney.net>

src/runtime/asm_arm.s

@@ -215,6 +215,9 @@ switch:
 	// save our state in g->sched.  Pretend to
 	// be systemstack_switch if the G stack is scanned.
 	MOVW	$runtime·systemstack_switch(SB), R3
+#ifdef GOOS_nacl
+	ADD	$4, R3, R3 // get past nacl-insert bic instruction
+#endif
 	ADD	$4, R3, R3 // get past push {lr}
 	MOVW	R3, (g_sched+gobuf_pc)(g)
 	MOVW	R13, (g_sched+gobuf_sp)(g)