runtime: fix MemStats on 32-bits

Int64's do not fit into uintptr's.

LGTM=khr
R=golang-codereviews, khr, rsc
CC=golang-codereviews, rlh
https://golang.org/cl/128380043

src/pkg/runtime/gc_test.go

@@ -164,6 +164,10 @@ func TestGcLastTime(t *testing.T) {
 	if t0 > last || last > t1 {
 		t.Fatalf("bad last GC time: got %v, want [%v, %v]", last, t0, t1)
 	}
+	pause := ms.PauseNs[(ms.NumGC+255)%256]
+	if pause == 0 || pause > 10e9 {
+		t.Fatalf("bad last GC pause: got %v, want [0, 10e9]", pause)
+	}
 }
 
 var hugeSink interface{}

src/pkg/runtime/malloc.go

@@ -477,11 +477,12 @@ func gogc(force int32) {
 			startTime = gonanotime()
 		}
 		// switch to g0, call gc, then switch back
-		mp.scalararg[0] = uint(startTime)
+		mp.scalararg[0] = uint(uint32(startTime)) // low 32 bits
+		mp.scalararg[1] = uint(startTime >> 32)   // high 32 bits
 		if force >= 2 {
-			mp.scalararg[1] = 1 // eagersweep
+			mp.scalararg[2] = 1 // eagersweep
 		} else {
-			mp.scalararg[1] = 0
+			mp.scalararg[2] = 0
 		}
 		onM(&gc_m)
 	}

src/pkg/runtime/mgc0.c

@@ -1401,8 +1401,8 @@ runtime·gc_m(void)
 	gp->status = Gwaiting;
 	gp->waitreason = "garbage collection";
 
-	a.start_time = g->m->scalararg[0];
-	a.eagersweep = g->m->scalararg[1];
+	a.start_time = (uint64)(g->m->scalararg[0]) | ((uint64)(g->m->scalararg[1]) << 32);
+	a.eagersweep = g->m->scalararg[2];
 	gc(&a);
 
 	gp->status = Grunning;