runtime: diagnose invalid pointers during GC

For #9880. Let's see what breaks.

Change-Id: Ic8b99a604e60177a448af5f7173595feed607875
Reviewed-on: https://go-review.googlesource.com/10818Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>

src/runtime/mbitmap.go

@@ -201,7 +201,7 @@ func heapBitsForObject(p uintptr) (base uintptr, hbits heapBits, s *mspan) {
 		// The following ensures that we are rigorous about what data
 		// structures hold valid pointers.
 		// TODO(rsc): Check if this still happens.
-		if false {
+		if true {
 			// Still happens sometimes. We don't know why.
 			printlock()
 			print("runtime:objectstart Span weird: p=", hex(p), " k=", hex(k))