Fix the code to use write barriers on heap memory, and no
write barriers on stack memory.

These errors were discoverd as part of fixing #27695. They may
have something to do with that issue, but hard to be sure.
The core cause is different, so this fix is a separate CL.

Update #27695

Change-Id: Ib005f6b3308de340be83c3d07d049d5e316b1e3c
Reviewed-on: https://go-review.googlesource.com/137438Reviewed-by: Austin Clements <austin@google.com>