Move to gcc for linking

If I'm not mistaken, the Clean optimizing linker does not support exporting a symbol table on Linux, which complicates the interworking features of https://gitlab.science.ru.nl/clean-and-itasks/abc-interpreter.

In my tests, /usr/bin/gcc:--gc-sections as EnvironmentLinker leads to executables of roughly the same size as lib/exe/linker. @johnvg, is there any reason why the optimizing linker should still be used?