runtime: move exit hooks into internal/runtime/exithook

runtime: move exit hooks into internal/runtime/exithook

This removes a //go:linkname usage in the coverage implementation.

For #67401.

Change-Id: I0602172c7e372a84465160dbf46d9fa371582fff
Reviewed-on: https://go-review.googlesource.com/c/go/+/586259
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>