runtime: fix freebsd build
notetsleep: nosplit stack overflow
        120	assumed on entry to notetsleep
        80	after notetsleep uses 40
        72	on entry to runtime.futexsleep
        16	after runtime.futexsleep uses 56
        8	on entry to runtime.printf
        -16	after runtime.printf uses 24

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12047043