Redesign scripts to build all images from a single branch

[mayeut]

As proposed in #542 (comment), this issue is created to propose and track progress of a rewrite of manylinux build scripts to build all images from a single branch.

With new manylinux policies allowed by PEP600, we'll have more and more images. Creating a branch per policy will cause issues for maintainability and consistency between images (or even README).

manylinux1 is out-of-scope for the rewrite for now (c.f. #542 (comment) for rational)

Here are the steps I propose, feel free to challenge those.
PRs referenced here will build one on top of the other requiring them to be reviewed sequentially.

• Rework the manylinux2014 branch to allow for multiple policies
  Rational for choosing this as the base branch of the rework: manylinux2014 branch builds on all platforms which allows to see issues that would otherwise not be seen until a later point.

  • Use graviton2 for arm64 build ([manylinux2014] Switch arm64 to using Travis-CI Graviton 2 #875)
  • Use a single Dockerfile for all images ([manylinux2014] Use a single Dockerfile for all images #876)
  • Rework scripts to be docker cache friendly (mostly for maintainers doing local builds)
    • Round 1 ([manylinux2014] Rework scripts to be more docker cache friendly #879)
    • Round 2 ([manylinux2014] Split libxcrypt build in runtime_base image #965)
    • Round 3 ([manylinux2014] Split patchelf build in runtime_base image #966)
    • Round 4 ([manylinux2014] Split libtool build in runtime_base image #967)
    • Round 5 ([manylinux2014] Split automake build in runtime_base image #968)
    • Round 6 ([manylinux2014] Split cmake build from build_base image #969)
    • Round 7 ([manylinux2014] Split git build from build_base image #970)
    • Round 8 ([manylinux2014] Split cpython build from build_base image #971)
    • Round 9 ([manylinux2014] Split sqlite3 build in build_cpython image #972)
    • Round 10 ([manylinux2014] Split cpython build by minor version #973)
    • Round 11 ([manylinux2014] Hardlink /opt/_internal to save 60MB #974)
    • Round 12 ([manylinux2014] Strip tools installed in runtime_base image #975)
    • Round 13 ([manylinux2014] Add swig #976)
    • Round 14 ([manylinux2014] Update autoconf from 2.69 to 2.70 #977)
    • Round 15 ([manylinux2014] Update automake from 1.16.2 to 1.16.3 #978)

• Merge manylinux2014 in master
  At this point, we'll be ready to add new images for newer PEP600 policies

  • Update glibc on manylinux2010 ([manylinux2010] glibc update & glibc cache/image size reduction #874)
  • Publish quay.io/pypa/manylinux2010_centos-6-no-vsyscall (Publish quay.io/pypa/manylinux2010_centos-6-no-vsyscall #878)
  • Add manylinux2010 build support in manylinux2014 branch ([manylinux2014] Allow manylinux2010 build from manylinux2014 branch #979)
  • Merge in master (Merge manylinux2014 branch in master branch #993)

• Add manylinux_2_24 images (Add manylinux_2_24 images #999)
  Rational for adding this specific policy: It solves issues for numpy ppc64le c.f. Tracking issue for PEP 600 Perennial manylinux rollout #542 (comment), numpy being the most downloaded manylinux package it seems a good starting point as it's also probably used as a dependency in a high number of manylinux packages which will thus benefit from this.

@pypa/manylinux, please do provide feedback as your time allows, thanks.