x/sys/plan9: function Exit causes build failure

package main

import "golang.org/x/sys/plan9"

func main() { plan9.Exit(0) }

This doesn't build

$ GOARCH=amd64 GOOS=plan9 go build
# test
golang.org/x/sys/plan9.exit: relocation target syscall.exit not defined

plan9.Exit calls plan9.exit, which is an assembly stub that jumps to syscall.exit. But there is no syscall.exit in the syscall package on Plan 9.

If the function is not needed (as it clearly doesn't work so nobody is using it), we probably can delete it. If it is needed for some reason, it could just call syscall.Exit, in Go code, not assembly.

cc @golang/plan9