runtime: hide the semaphore table from garbage collector
runtime: hide the semaphore table from garbage collector

R=rsc, dvyukov, dave
CC=golang-dev
http://codereview.appspot.com/6528043

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang