reflect: use correct write barrier operations for method funcs

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/137438


Reviewed-by: Austin Clements <austin@google.com>