runtime: fix Plan 9 build

The current failures were:

fatal error: runtime: stack split during syscall
goroutine 1 [stack split]:
runtime.findnull(0x105a9080)
        /usr/go/src/pkg/runtime/string.goc:14 fp=0x305aefb8
runtime: unexpected return pc for runtime.errstr called from 0x80
runtime.errstr()
        /usr/go/src/pkg/runtime/sys_plan9_386.s:196 +0x2f fp=0x305aefc8

fatal error: runtime: stack split during syscall
goroutine 2 [stack split]:
runtime.nanotime(0x305bff3c)
        /usr/go/src/pkg/runtime/time_plan9_386.c:9 fp=0x305bff34
notetsleep(0x305bff9c, 0xf8475800, 0xd, 0x0, 0x0, ...)
        /usr/go/src/pkg/runtime/lock_sema.c:195 +0x87 fp=0x305bff48
runtime.notetsleepg(0x305bff9c, 0xf8475800, 0xd)
        /usr/go/src/pkg/runtime/lock_sema.c:266 +0xa4 fp=0x305bff68
runtime.MHeap_Scavenger()
        /usr/go/src/pkg/runtime/mheap.c:463 +0xc2 fp=0x305bffd0
runtime.goexit()
        /usr/go/src/pkg/runtime/proc.c:1332 fp=0x305bffd4
created by runtime.main
        /usr/go/src/pkg/runtime/proc.c:168

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12128043

Committer: Brad Fitzpatrick <bradfitz@golang.org>