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/asm_386.s

@@ -2146,3 +2146,6 @@ TEXT runtime·duffcopy(SB), NOSPLIT, $0-0
 	ADDL	$4,DI
 	
 	RET
+
+TEXT runtime·timenow(SB), NOSPLIT, $0-0
+	JMP	time·now(SB)

src/pkg/runtime/asm_amd64.s

@@ -2179,3 +2179,6 @@ TEXT runtime·duffcopy(SB), NOSPLIT, $0-0
 	ADDQ	$8,DI
 
 	RET
+
+TEXT runtime·timenow(SB), NOSPLIT, $0-0
+	JMP	time·now(SB)

src/pkg/runtime/asm_amd64p32.s

@@ -1064,3 +1064,6 @@ TEXT bytes·Equal(SB),NOSPLIT,$0-25
 eqret:
 	MOVB	AX, ret+24(FP)
 	RET
+
+TEXT runtime·timenow(SB), NOSPLIT, $0-0
+	JMP	time·now(SB)

src/pkg/runtime/asm_arm.s

@@ -747,3 +747,6 @@ _sib_notfound:
 	MOVW	$-1, R0
 	MOVW	R0, ret+12(FP)
 	RET
+
+TEXT runtime·timenow(SB), NOSPLIT, $0-0
+	B	time·now(SB)

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

src/pkg/runtime/mgc0.c

@@ -91,6 +91,8 @@ enum {
 // Initialized from $GOGC.  GOGC=off means no gc.
 static int32 gcpercent = GcpercentUnknown;
 
+void runtime·gc_unixnanotime(int64 *now);
+
 static FuncVal* poolcleanup;
 
 void
@@ -2404,7 +2406,7 @@ gc(struct gc_args *args)
 	mstats.next_gc = mstats.heap_alloc+mstats.heap_alloc*gcpercent/100;
 
 	t4 = runtime·nanotime();
-	mstats.last_gc = t4;
+	runtime·gc_unixnanotime((int64*)&mstats.last_gc);  // must be Unix time to make sense to user
 	mstats.pause_ns[mstats.numgc%nelem(mstats.pause_ns)] = t4 - t0;
 	mstats.pause_total_ns += t4 - t0;
 	mstats.numgc++;

src/pkg/runtime/mgc0.go

@@ -18,3 +18,10 @@ func gc_g_ptr(ret *interface{}) {
 func gc_itab_ptr(ret *interface{}) {
 	*ret = (*itab)(nil)
 }
+
+func timenow() (sec int64, nsec int32)
+
+func gc_unixnanotime(now *int64) {
+	sec, nsec := timenow()
+	*now = sec*1e9 + int64(nsec)
+}