OpenBSD 6.4+ requires that stack regions be marked with MAP_STACK.
MAP_STACK is permitted (but is a no-op) on OpenBSD 6.3.

Change-Id: I8118f27e6a0feb97ae570cbc8aad461ec20676fa
Reviewed-on: https://go-review.googlesource.com/c/154477
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>