runtime: add race detector tips to reportZombies func

We can find a few issues finally turned out to be a race condition,
such as #47513. I believe such a tip can eliminate the need for developers
to file this kind of issue in the first place.

Change-Id: I1597fa09fde641882e8e87453470941747705272
GitHub-Last-Rev: 9f136f5
GitHub-Pull-Request: #70331
Reviewed-on: https://go-review.googlesource.com/c/go/+/627816
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Jorropo <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: linsite

src/runtime/mgcsweep.go

@@ -855,7 +855,7 @@ func (sl *sweepLocked) sweep(preserve bool) bool {
 // pointer to that object and marked it.
 func (s *mspan) reportZombies() {
 	printlock()
-	print("runtime: marked free object in span ", s, ", elemsize=", s.elemsize, " freeindex=", s.freeindex, " (bad use of unsafe.Pointer? try -d=checkptr)\n")
+	print("runtime: marked free object in span ", s, ", elemsize=", s.elemsize, " freeindex=", s.freeindex, " (bad use of unsafe.Pointer or having race conditions? try -d=checkptr or -race)\n")
 	mbits := s.markBitsForBase()
 	abits := s.allocBitsForIndex(0)
 	for i := uintptr(0); i < uintptr(s.nelems); i++ {