[dev.power64] runtime: Fix broken merge of noasm.go

The earlier dev.power64 merge missed the port of
runtime/noasm.goc to runtime/noasm_arm.go.  This CL fixes this
by moving noasm_arm.go to noasm.go and adding a +build to
share the file between arm and power64.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/158350043