Add native linux- and darwin-aarch64 tests

[mrdomino]

Removes the test-cross target for aarch64-unknown-linux-gnu, preferring to run this test against GitHub’s native Linux arm runners. Should give a considerable speedup from the combination of not needing to be virtualized and not needing to set up the cross environment.

Also adds a test case against Darwin on macos-latest to get another aarch64 platform, and avoids running an empty deps action if there is none.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.87%. Comparing base (59b58ec) to head (0e886c8).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1027   +/-   ##
=======================================
  Coverage   79.87%   79.87%           
=======================================
  Files         163      163           
  Lines       17737    17737           
=======================================
  Hits        14167    14167           
  Misses       3570     3570           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tarcieri]

Nice, though it didn't seem to have an overall impact on the test suite time, but it should help free up runners faster, which should help with concurrent test runs

[mrdomino]

Yeah, that's probably still dominated by the powerpc cross test. I wonder if there's a faster way to get confirmation that it works against big-endian systems.

[tarcieri]

Some of the longest tests time-wise are actually the i686 tests, because a considerable amount of time is spent installing gcc-multilib, which is a general problem across @RustCrypto.

It sure would be nice if it were pre-installed on the GitHub Actions runners