all: make linux/arm64 a first-class port

[bradfitz]

Currently, linux/arm is a "first class port" but linux/arm64 is not.

At this point, arm64 is probably equally or more important than arm.

I think we should add linux/arm64 to the first class port set.

[rsc]

There seems to be a strong consensus that we should do this, so it's a likely accept.

Leaving open for a week for final comments.

[rsc]

No change in consensus, so accepting.

[bradfitz]

Updated the wiki page. Closing.

(#27689 remains open about figuring out the source of truth and/or synchronizing this information, but for now the wiki is canonical)