Add to tox.ini 32-bit linux environments and other architectures supported by docker

[mkoeppe]

#29053 adds tox.ini to support testing the sage distribution in isolated environments via docker.

This ticket adds linux environments for various other architectures supported by docker (see ​https://github.com/docker-library/official-images#architectures-other-than-amd64).

Examples:

$ tox -e docker-fedora-31-standard-arm64v8
$ tox -e docker-debian-buster-standard-i386
$ tox -e docker-ubuntu-latest-standard-s390x

This works out of the box with macOS Docker Desktop because it has binfmt_misc with qemu enabled.

• How to Build and Run ARM Docker Containers on x86 Hosts · MatchboxBlog? - ​https://matchboxdorry.gitbooks.io/matchboxblog/content/blogs/build_and_run_arm_images.html
• Cross Building and Running Multi-Arch Docker Images – Micheal Waltz – DevOps? Engineer - ​https://www.ecliptik.com/Cross-Building-and-Running-Multi-Arch-Docker-Images/
• multiarch/qemu-user-static - Docker Hub - ​https://hub.docker.com/r/multiarch/qemu-user-static/
• multiarch/ubuntu-core - Docker Hub - ​https://hub.docker.com/r/multiarch/ubuntu-core/

[mkoeppe]

on top of #29104 to avoid merge conflicts

[mkoeppe]

Last 10 new commits:

​98edff6	Add build/pkgs/gfortran/distros/fedora.txt
​843ca76	build/pkgs/fedora.txt: Add python3
​bd6a545	tox.ini: Add debian stretch, put sid last
​23c6334	build/pkgs/openblas/distros/arch.txt: Add lapack, cblas
​fca6654	tox.ini: Add toxenvs local-homebrew-macos-minimal, local-homebrew-macos-standard
​e0e8a4c	tox.ini [local-homebrew]: Fixup PATH, PKG_CONFIG_PATH, set LDFLAGS, CPPFLAGS; [local-direct]: New
​16ca88f	tox.ini: Use {envlogdir}; use ln -sf
​6a1803d	tox.ini [local]: First build the base-toolchain
​4b72987	tox.ini [local-homebrew]: Set PATH earlier and set ACLOCAL_PATH for bootstrapping
​839d6fb	tox.ini: Add other archs supported by docker

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​67e14ce

fixup for llocal

[mkoeppe]

This works on macOS because it is running everything in a virtualbox.

I haven't succeeded in getting it to work on github actions yet. last attempt: ​https://github.com/mkoeppe/sage/runs/422184737?check_suite_focus=true