runtime: correct FixedStack value (fixes windows build)
runtime: correct FixedStack value (fixes windows build)

Fixes issue 2068.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4705046

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