cmd/go/internal/telemetrystats: count cgo usage

cmd/go/internal/telemetrystats: count cgo usage

Knowing how many times cgo is used is useful information to have in the
local telemetry database.

It also opens the door for uploading them in the future if desired.

Change-Id: Ia92b11fc489f015bbface7f28ed5a5c2871c44f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/707055
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>