runtime: scan defer closure in stack scan

With stack objects, when we scan the stack, it scans defers with
tracebackdefers, but it seems to me that tracebackdefers doesn't
include the func value itself, which could be a stack allocated
closure. Scan it explicitly.

Alternatively, we can change tracebackdefers to include the func
value, which in turn needs to change the type of stkframe.

Fixes #30453.

Change-Id: I55a6e43264d6952ab2fa5c638bebb89fdc410e2b
Reviewed-on: https://go-review.googlesource.com/c/164118


Reviewed-by: Keith Randall <khr@golang.org>