Rollup merge of #161562 - Mark-Simulacrum:alt-ec2, r=Kobzol

More EC2 instance usage

This extends our usage of EC2 to dist-x86_64-linux-alt builders (1h23m with m8a.2xlarge, https://github.com/rust-lang/rust/actions/runs/32137297331/job/95711706485).

This is moving from GHA credits to EC2 credits, so we should confirm we want that, but in dollar terms this is cheaper: at 1h55m (last auto build) on [GHA $1.32/hr](https://docs.github.com/en/billing/reference/actions-runner-pricing) = $2.53/run, vs. 1h23m on EC2 at $0.48688/hr = $0.67/run.

My primary goal is to try to free up GHA credits so we can move Windows and/or macOS jobs to large runners, since I suspect us trying to host those ourselves is going to be more painful.

As a drive-by change this also adds support for EC2 aarch64 machines to the bors config (but not CI config). My suspicion is that if/when we have aarch64 perf we may want faster aarch64 try builds, and in any case dist-aarch64-linux is one of our slower runners -- at 2h24m -- so it may benefit from getting a faster machine. I don't know yet how EC2 will compare but extending the bors config is cheap so I'd rather just do that now to enable easier testing.

I also deleted some of the old unused runner templates, I don't see much point in keeping dead ones around.

r? Kobzol