Consider removing old C & C++ compilers (gcc48, gcc49, clang_5)

`nixpkgs` has some pretty old C and C++ compilers, particularly `gcc48`, `gcc49`, and `clang_5`.

Afaict none of them are used anywhere.

There was an attempt to remove `gcc49` in #77985, but it was ultimately kept. One argument in favor of keeping the gccs around was `mobile-nixos`, but that does not seem to use them any more.