Rollup merge of #146453 - thejpster:arm-linux-docs, r=petrochenkov

Add general arm-linux.md platform doc.

Adds a new page that covers all 32-bit Arm Linux systems. This means that we can reduce the amount of information required in the target specific pages to just the Tier level, the maintainer, and any specific details for that target.

I have no changed those pages yet, though. Let's start with this.

Author: matthiaskrgr