runtime: use explicit flag when finalizer goroutine is waiting
runtime: use explicit flag when finalizer goroutine is waiting

Avoids spurious wakeups during other sleeping by that goroutine.
Fixes issue 711.

R=r
CC=golang-dev
http://codereview.appspot.com/902041

ssrl/go,gnanderson/go,tianon/debian-golang,nightlyone/golang,ssrl/go,h8liu/golang,tv42/old-go,h8liu/golang,bdwalton/debian-golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,ssrl/go,tianon/debian-golang,gnanderson/go,h8liu/golang,pombredanne/go-deleteme,ssrl/go,nightlyone/golang,h8liu/golang,tianon/debian-golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,nightlyone/golang,tv42/old-go,apparentlymart/golang,ssrl/go,h8liu/golang,apparentlymart/golang,dsriram/golang,tianon/debian-golang,ssrl/go,tianon/debian-golang,h8liu/golang,bdwalton/debian-golang,tv42/old-go,h8liu/golang,bdwalton/debian-golang,dsriram/golang,nightlyone/golang,tianon/debian-golang,bdwalton/debian-golang,apparentlymart/golang,tv42/old-go,bdwalton/debian-golang,bdwalton/debian-golang,dsriram/golang,apparentlymart/golang,nightlyone/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,bdwalton/debian-golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,nightlyone/golang,nightlyone/golang,bdwalton/debian-golang,tv42/old-go,tv42/old-go,dsriram/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tianon/debian-golang,ssrl/go,tianon/debian-golang,apparentlymart/golang