runtime: reenable bad pointer check in GC

The last time we tried this, linux/arm64 broke.
The series of CLs leading to this one fixes that problem.
Let's try again.

Fixes #9880.

Change-Id: I67bc1d959175ec972d4dcbe4aa6f153790f74251
Reviewed-on: https://go-review.googlesource.com/12849Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: 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))