runtime: bump gc 'extra bytes' check
runtime: bump gc 'extra bytes' check

(needed for non-zero GOMAXPROCS)

R=iant, rsc
CC=go.peter.90, golang-dev
http://codereview.appspot.com/5486059

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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