runtime: fix darwin build Add place holder for runtime.osyield.
runtime: fix darwin build
Add place holder for runtime.osyield.

R=golang-dev
CC=golang-dev
http://codereview.appspot.com/5149049

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