encoding/gob: change typeInfo.encoder type to atomic.Pointer[T]

encoding/gob: change typeInfo.encoder type to atomic.Pointer[T]

Replace loading and storing an atomic.Value of type pointer with
atomic.Pointer.

Change-Id: I018ac1e18eee4f203ebca65c2833daf991075371
Reviewed-on: https://go-review.googlesource.com/c/go/+/422174
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Rob Pike <r@golang.org>