runtime: move getclosureptr to internal/runtime/sys

runtime: move getclosureptr to internal/runtime/sys

Moving these intrinsics to a base package enables other internal/runtime
packages to use them.

There is no immediate need for getclosureptr outside of runtime, but it
is moved for consistency with the other intrinsics.

For #54766.

Change-Id: Ia68b16a938c8cb84cb222469db28e3a83861be5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/613262
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>