Check free and top availability only on Windows

The regression has been introduced in 7c98912. procps is unavailable
on OS X and the script already reflects that by using sysctl directly.
On the vast majority of Linux distributions both tools are provided
either by procps or busybox.

Change-Id: I222beab7083399b7bf74c9560391da8aa65b7348
Closes-Bug: 1394141
Merge "Check free and top availability only on Windows"