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 issue 5685.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/13234050
build: remove various uses of C undefined behavior

If you thought gcc -ansi -pedantic was pedantic, just wait
until you meet clang -fsanitize=undefined.

I think this addresses all the reported "errors", but we'll
need another run to be sure.

all.bash still passes.

Update issue 5764

Dave, can you please try again?

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13334049