runtime: account for tiny allocs, for testing.AllocsPerRun

Fixes issue 8734.

LGTM=r, bradfitz, dvyukov
R=bradfitz, r, dvyukov
CC=golang-codereviews, iant, khr
https://codereview.appspot.com/143150043
doc/go1.4.txt: add tinyallocs change

CC=golang-codereviews
https://codereview.appspot.com/145050043