[simple-system] Linker script updates

[tomeroberts]

There are quite a few sections unspecified at the moment (mostly debug related). Adding them would allow linking with the --orphan-handling=error option to check for accidentally missed ones (that we care about).

We could also specify __global_pointer$ for possible optimization.

[lenary]

I know @silvestrst has been wanting to do more of this kind of work (linker script and assembly-level programming) - if this isn't urgent, this seems a useful place he can contribute to Ibex?