{main,cross}: Restore gcc4/gcc6 for v21.12 stable branch

Compilers are a big pain to build, and right now everyone who wants to test the stable branch on some unsupported downstream devices has to build gcc4/6 themselves locally, wasting lots of precious energy. :)

Having these (cross-)compilers available in the postmarketOS repo for stable releases can be also useful for other projects. For example, for lk2nd I would like to do some basic compile testing with gcc4 (since this is what Qualcomm used). The gcc4 from postmarketOS is pretty much the most easily available gcc4-armv7 for Alpine Linux. :)

This should not have any disadvantage for devices using stable, there will just be additional packages available in the binary repository.
This commit simply reverts parts of b78a0ca7 ("remove packages not supported in v21.12").

[ci:skip-build]: already built locally (successfully), will probably take too long