runtime: re-enable preemption

Update #543

I believe the runtime is strong enough now to reenable
preemption during the function prologue.
Assuming this is or can be made stable, it will be in Go 1.2.
More aggressive preemption is not planned for Go 1.2.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/11433045

src/pkg/runtime/proc.c

@@ -2175,12 +2175,6 @@ preemptone(P *p)
 	M *mp;
 	G *gp;
 
-// Preemption requires more robust traceback routines.
-// For now, disable.
-// The if(1) silences a compiler warning about the rest of the
-// function being unreachable.
-if(1) return;
-
 	mp = p->m;
 	if(mp == nil || mp == m)
 		return;