Run aarch64-pc-windows-msvc tests on ARM64 Windows runners

[dpaoliello]

We recently got ARM64 Windows runner enabled, so now we should be able to run the aarch64-pc-windows-msvc tests on them.

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[dpaoliello]

@rustbot blocked Waiting on ARM64 runners to be enabled for this repo: https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/GitHub.20Arm64.20runners.20private.20preview/near/444523895

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[bors]

:umbrella: The latest upstream changes (presumably 87158e666a6b290fab74a97ac3393623969d09c9) made this pull request unmergeable. Please resolve the merge conflicts.

[dpaoliello]

This was already added by https://github.com/rust-lang/stdarch/pull/1785