sections: add src_root to verilator build include

This solves a problem with inter-core dependencies,
where header files have to be shared between cores.
Doing it this way, cores can include files from other cores by:
#include <other-core/file.h>

The alternative would be for each core to export an include
path, but this might result in namespace clashes between include
files from different cores. Plus, it would be messier to
implement in fusesoc.
verilator: use --start-group/--end-group on archives only

I don't really have an excellent rational behind this,
other than it fixes a real-world problem I had when compiling
several cores depending on each other, and it doesn't seem to
break anything else.
sections: add CPP source_type