runtime: remove OABI check from ARM startup

The code in question is trying to print a nice error message
when a Go EABI binary runs on an OABI machine.
Unfortunately, the only way to do that is to use
ARM Thumb instructions, which we otherwise don't use.

There exist ARM EABI machines that do not support Thumb.
We could run on them if not for this OABI check, so disable it.

Fixes #5685.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/13234050