We are seeing a bad stack map in #18190. In a copystack, it is
mistaking a slot for a pointer.

Presumably this is caused either by our fledgling dynlink support on
darwin, or a consequence of having two copies of the runtime in the
process. But I have been unable to work out which in the 1.8 window,
so pushing darwin support to 1.9 or later.

Change-Id: I7fa4d2dede75033d9a428f24c1837a4613bd2639
Reviewed-on: https://go-review.googlesource.com/34391Reviewed-by: Ian Lance Taylor <iant@golang.org>