runtime: revise openbsd/arm runtime defs

The OpenBSD arm port switched to EABI in September 2016 - this revises the layout
of the runtime definitions to match what the kernel currently uses.

Change-Id: I1bca7de56979f576862a7c280631e835f7ae4278
Reviewed-on: https://go-review.googlesource.com/c/153577


Reviewed-by: Ian Lance Taylor <iant@golang.org>