Add CI on Github Actions for x86_64, aarch64, arm, ppc64 and s390x (#476)

* Add CI on Github Actions for x86_64, aarch64, armhf, ppc64 and s390x
* Disable -march=native for testervecabi - fails when compiling on a host CPU with AVX512f (#478)
* Enable qemu features for s390x, ppc64, and arm
* Add some architecture-specific cmake flags
* Use job dependencies to avoid duplicating building native
* Fix permissions in downloaded artifacts
* Disable arm inline headers - don't work (#480)
* Document why DISABLE_VXE2 on s390x