runtime: move atoi to internal/runtime/strconv

runtime: move atoi to internal/runtime/strconv

Moving to a smaller package allows its use in other internal/runtime
packages.

This isn't internal/strconvlite since it can't be used directly by
strconv.

For #73193.

Change-Id: I6a6a636c9c8b3f06b5fd6c07fe9dd5a7a37d1429
Reviewed-on: https://go-review.googlesource.com/c/go/+/672697
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>