provide nightly rust-std for aarch64-pc-windows-msvc target

[froydnj]

rustup doesn't appear to know about rust-std for aarch64-pc-windows-msvc, and poking around on static.rust-lang.org suggests that this component isn't being built. #53173 added the appropriate target to rustc, and it'd be great to continue that work by making the standard library readily available.

I'm happy to make the changes myself, but I couldn't find the obvious place to change things to ensure things are built on the Windows builders...

/cc @alexcrichton

[alexcrichton]

Ah yes currently this isn't automatically built on CI. I think last I attempted this it failed to build libtest, so we probably can't add it to CI yet until that's fixed unfortunately :(

Once that's done though we'd need to modify our AppVeyor config to build the target. Hopefully not too hard to do! We may be a bit strapped for time on CI but we can figure out somewhere to squeeze it in likely.