runtime: make MemStats.LastGC Unix time again

The monotonic clock patch changed all runtime times
to abstract monotonic time. As the result user-visible
MemStats.LastGC become monotonic time as well.
Restore Unix time for LastGC.

This is the simplest way to expose time.now to runtime that I found.
Another option would be to change time.now to C called
int64 runtime.unixnanotime() and then express time.now in terms of it.
But this would require to introduce 2 64-bit divisions into time.now.
Another option would be to change time.now to C called
void runtime.unixnanotime1(struct {int64 sec, int32 nsec} *now)
and then express both time.now and runtime.unixnanotime in terms of it.

Fixes #7852.

LGTM=minux.ma, iant
R=minux.ma, rsc, iant
CC=golang-codereviews
https://golang.org/cl/93720045

src/pkg/runtime/gc_test.go

@@ -9,6 +9,7 @@ import (
 	"runtime"
 	"runtime/debug"
 	"testing"
+	"time"
 )
 
 func TestGcSys(t *testing.T) {
@@ -152,6 +153,18 @@ func TestGcRescan(t *testing.T) {
 	}
 }
 
+func TestGcLastTime(t *testing.T) {
+	ms := new(runtime.MemStats)
+	t0 := time.Now().UnixNano()
+	runtime.GC()
+	t1 := time.Now().UnixNano()
+	runtime.ReadMemStats(ms)
+	last := int64(ms.LastGC)
+	if t0 > last || last > t1 {
+		t.Fatalf("bad last GC time: got %v, want [%v, %v]", last, t0, t1)
+	}
+}
+
 func BenchmarkSetTypeNoPtr1(b *testing.B) {
 	type NoPtr1 struct {
 		p uintptr