Improve debugging documentation and scripts

[CharlyCst]

We had some issue with getting GDB to work across platforms, causing errors in our just gdb command.

Currently just gdb starts rust-gdb, which is simply a wrapper around the default gdb of the platform. The problem is that the default gdb varies across platforms:

• On Intel Mac it is gdb-multiarch, which works with RISC-V
• On older Ubuntu it is gdb which only works for the host platform architecture, so it doesn't work with RISC-V
• On Arm Mac gdb is not available for the host architecture, so the user must manually install riscv64-elf-gdb

I am not sure what the best solution is to make it works seamlessly for everyone, maybe trying to detect if gdb-multiarch or riscv64-elf-gdb are installed and pick the available one or display an helpful error message?