move enhance_call_tree_pattern to gcc_tools, and teach gcc_tools how to pick up call trees for RISC-V

[jwise]

I think enhance_call_tree_pattern is the only bit of arch-specific code. Probably in an ideal universe it would go into some arch/ library, but for now, gcc_tools is the one that knows what arch we're targeting (sort of), so I move that pattern there. Then, I add support for branch patterns for RISC-V. This allows puncover to track stack usage on RV32 / LiteX.