runtime: fix race condition between GC and setGCPercent If first GC runs concurrently with setGCPercent, it can overwrite gcpercent value with default.
runtime: fix race condition between GC and setGCPercent
If first GC runs concurrently with setGCPercent,
it can overwrite gcpercent value with default.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/10242047

gnanderson/go,apparentlymart/golang,tv42/old-go,gnanderson/go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme