runtime: add reference to debugPinnerV1

runtime: add reference to debugPinnerV1

This is intended to be used by debuggers, to keep heap memory reachable
even if it isn't referenced from anywhere else.

Change-Id: I1e900e02b4fe3a188f8173cec70f8de32122489b
Reviewed-on: https://go-review.googlesource.com/c/go/+/682875
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Mark Freeman <mark@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>