Currently freeOSMemory makes only marking phase of GC, but not sweeping phase.
So recently memory is not released after freeOSMemory.
Do both marking and sweeping during freeOSMemory.
Fixes #8019.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rsc
https://golang.org/cl/97550043