Add support for aarch64-unknown-linux-gnu

[aledomu]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.17%. Comparing base (ddca0b8) to head (9214687).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1241      +/-   ##
==========================================
- Coverage   46.20%   46.17%   -0.04%     
==========================================
  Files         186      186              
  Lines       66139    66138       -1     
==========================================
- Hits        30561    30537      -24     
- Misses      35578    35601      +23     

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[aledomu]

This requires work on the CI/CD side as with the musl variant:

Unable to find image 'tectonictypesetting/crossbuild:aarch64-unknown-linux-gnu' locally
docker: Error response from daemon: manifest for tectonictypesetting/crossbuild:aarch64-unknown-linux-gnu not found: manifest unknown: manifest unknown.

[CraftSpider]

The CI images currently come from here:
https://github.com/tectonic-typesetting/tectonic-ci-support/tree/master/cross-images

They're based on very old versions of the original cross containers, as I understand, with the additional dependencies required by Tectonic added on top.

[aledomu]

Since this is far easier to do for the maintainer than myself and I don't need this anymore nor I see any interest from the maintainer, I'm closing this.