Write platform support docs for aarch64-unknown-none and aarch64-unknown-none-softfloat #859
Description
The Rust Compiler Team plans to begin enforcing Tier 2 target requirements: see #851. We would like to keep aarch64-unknown-none and aarch64-unknown-none-softfloat at Tier 2, but they need Platform Support docs. The Platform Support section of rust-lang/rust contains instructions; probably could start with thumbv7em-none-eabi or similar and do edits.