runtime: background goroutines show up in early failures

[rsc]

I don't ever want to see these goroutines.
We used to mark them as system goroutines while creating them.
Now they get marked as system when they start running, so there is a race: if the failure happens before they get a chance to run, they show up.

We need to get rid of the race.

g% new6g
panic: flag

goroutine 1 [running]:
cmd/internal/obj.Flagcount(0x2a0f50, 0x1, 0x2d1c70, 0x11, 0x421c30)
/Users/rsc/g/go/src/cmd/internal/obj/flag.go:7 +0x64
cmd/internal/gc.Main()
/Users/rsc/g/go/src/cmd/internal/gc/lex.go:178 +0xcdd
main.main()
/Users/rsc/g/go/src/cmd/new6g/galign.go:108 +0x56a

goroutine 2 [runnable]:
runtime.forcegchelper()
/Users/rsc/g/go/src/runtime/proc.go:111
runtime.goexit()
/Users/rsc/g/go/src/runtime/asm_amd64.s:2454 +0x1

goroutine 3 [runnable]:
runtime.runfinq()
/Users/rsc/g/go/src/runtime/mfinal.go:115
runtime.goexit()
/Users/rsc/g/go/src/runtime/asm_amd64.s:2454 +0x1
g%