runtime: fix openbsd build

runtime: fix openbsd build
notetsleep: nosplit stack overflow
        120	assumed on entry to notetsleep
        96	after notetsleep uses 24
        88	on entry to runtime.semasleep
        32	after runtime.semasleep uses 56
        24	on entry to runtime.nanotime
        -8	after runtime.nanotime uses 32

Nanotime seems to be using only 24 bytes of stack space.
Unless I am missing something.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12041044