[dev.typeparams] Revert "[dev.typeparams] runtime: make deferproc take a func() argument"

Temprary revert CL 325918.

Delve relies on the _defer.fn.fn field to get defer frames.
CL 325918 changes the type of _defer.fn to func(), which no
longer has an fn field.

Change-Id: If6c71b15a27bac579593f5273c9a49715e6e35b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/327775


Trust: Cherry Mui <cherryyz@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>