getcallerpc's arg needs to point to the first argument slot.
I believe this bug was introduced by Michel's itab changes
(specifically https://go-review.googlesource.com/c/20902).

Fixes #15145

Change-Id: Ifb2e17f3658e2136c7950dfc789b4d5706320683
Reviewed-on: https://go-review.googlesource.com/21931Reviewed-by: Michel Lespinasse <walken@google.com>