link: restructure linking to work with old toolchain

As announced, this should hopefully make Sytare compile again with the old toolchain. Restructuring like this has another benefit for the future. Doing it like this we will be able to omit the size definitions of the data/bss sections. This will allow for more flexible memory usage (say, you don't use many drivers but a lot of space in .usr_data) and extend automatically for CPUs with more RAM.