Platform Support: Libs teams relationship with target maintainers

[yaahc]

Building upon the Target tier policy the libs team would like to more clearly define our relationship with target maintainers and ensure that we are providing support as necessary to them and the work they do within the rust project to maintain support for rust on their respective platforms.

This was brought up in the context of this PR modifying system library linkage on netbsd. In the process of reviewing that PR I realized that we don't have a designated maintainer for netbsd or for many other platforms which got grandfathered in to the new system.

After a discussion with the rest of the libs team we've resolved to work to improve this situation, and we've agreed on the following steps.

• Open issues for existing platforms that do not have designated maintainers to identify and document existing maintainers, recruit new maintainers, or, if all of that fails, mark the platforms as as unmaintained
• Create associated ping groups for "target experts" to notify platform maintainers and contributors of changes to their platform so they can participate in those discussions and provide reviews

This came up again in another issue where a new feature was introduced but existing maintainers were not notified of it or given a chance to implement it prior to stabilization. We will be discussing this issue further in the next libs team meeting but I will be recommending that we additionally institute the following changes to our process:

• Setup general ping groups for each target tier
• Add "Ping tier2 and tier3 maintainers" as a required step to new tracking issues and again during stabilization for issues introducing features that require platform specific implementations