Target tier policy

[joshtriplett]

Rust developers regularly implement new targets in the Rust compiler,
and reviewers of pull requests for such new targets would like a clear,
consistent policy to cite for accepting or rejecting such targets.
Currently, individual reviewers do not know what overall policy to
apply, and whether to apply solely their own judgment or defer to a Rust
governance team.

Rust developers regularly ask how they can raise an existing target to
tier 2 (and in particular how they can make it available via rustup),
and occasionally ask what it would take to add a new tier 1 target. The
Rust project has no clear official policy for target tiers. People not
only don't know, they don't know who to ask or where to start.

This proposal documents an official policy for adding new (tier 3)
targets, and for raising targets to tier 2 (with rustup builds) or to
tier 1.

Based on discussions with the compiler team and representatives from
other teams.

Note: when merging this RFC, please don't squash-merge.
This policy has gone through substantial revisions, and the history of those revisions would be useful.

Rendered