runtime: enable goroutine preemption

All known issues with preemption have beed fixed.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12008044

src/pkg/runtime/proc.c

@@ -2266,7 +2266,7 @@ preemptone(P *p)
 // For now, disable.
 // The if(1) silences a compiler warning about the rest of the
 // function being unreachable.
-if(1) return;
+if(0) return;
 
 	mp = p->m;
 	if(mp == nil || mp == m)

src/pkg/runtime/proc_test.go

@@ -193,7 +193,6 @@ var preempt = func() int {
 }
 
 func TestPreemption(t *testing.T) {
-	t.Skip("preemption is disabled")
 	// Test that goroutines are preempted at function calls.
 	const N = 5
 	c := make(chan bool)
@@ -214,7 +213,6 @@ func TestPreemption(t *testing.T) {
 }
 
 func TestPreemptionGC(t *testing.T) {
-	t.Skip("preemption is disabled")
 	// Test that pending GC preempts running goroutines.
 	const P = 5
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(P + 1))