Rollup merge of #90498 - joshtriplett:target-tier-policy-draft-updates, r=Mark-Simulacrum

Clarifications in the target tier policy

We've added several targets since the introduction of the target tier policy. Based on experiences of those adding such targets, and discussions around such additions, clarify the target tier policy to make it easier to follow and work with.

None of these changes substantively change the requirements on targets. (In some cases the changes do direct target submitters to follow specific process requirements for the addition of a target, such as how to respond to requirements, where to put target-specific documentation, or what should appear in that documentation. Those changes are procedural in nature and document the procedures we already direct people to follow.)

- Clarify how to quote and respond to the target tier policy requirements. Several times, people have seemed unclear on how to respond to some of the policy requirements, particularly those that just state things the target developers must *not* do (e.g. not posting to PRs that break the target). Add a note that such requirements just need acknowledgement, nothing more.
- Clarify dependency requirements in the face of cross-compilation. I previously phrased this confusingly in terms of "host tools", since that is the case where an exception applies (allowing proprietary target libraries commonly used by binaries for the target). Rephrase it to apply equally to cross-compilation. This doesn't change the net effect of the requirements, since other requirements already cover the dependencies of the Rust toolchain.
- Clarify documentation about running binaries. The requirement for target documentation talks about "running tests", but tier 3 targets often don't support running the full testsuite, and in practice the documentation for how to run an individual binary may be more useful. Change "running tests" to "running binaries, or running tests".
- Explain where to place target-specific documentation (a subdirectory of platform-support, with a link from the platform-support entry for the target).
- Add a template for target-specific documentation.

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