ARM's udiv function is nosplit and it shouldn't be preemptied
(passing args in registers). It is in some sense like DUFFCOPY,
which we don't mark as safepoint.

Change-Id: I49f7c4e69e787ac364d0b0def0661e79a0ea9e69
Reviewed-on: https://go-review.googlesource.com/41370
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>