runtime: fix runtime.Breakpoint on ARMv5

Fixes issue 8775.

Use the illegal instruction suggested by Ian in https://codereview.appspot.com/144180043/#msg4 on all arm arches.

LGTM=minux
R=golang-codereviews, gobot, rsc
CC=golang-codereviews, iant, minux
https://codereview.appspot.com/146130043