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/12849


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>