internal/runtime/cgroup: allow more tests to run on all OSes

Move non-Linux specific part out of _linux.go. The parsing code itself
doesn't depend anything Linux specific, even if it the format is Linux
specific.

This should benefit developers working on non-Linux OSes.

Change-Id: I1692978d583c3dd9a57ff269c97e8fca53a7a057
Reviewed-on: https://go-review.googlesource.com/c/go/+/723240
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Carrillo Rodriguez <carrillorodriguez672@gmail.com>

Author: huww98