spike-dasm: Add Snitch ISA aware riscv-isa-sim as submodule

[GiannaP]

Add Snitch ISA aware riscv-isa-sim version as submodule. Only used for spike-dasm to make the generated traces more readable.

In the old snitch repository it was flattened (without vendor) from the repository of @huettern. The branch was ported to the pulp-platform repository.

[colluca]

We should adapt the Dockerfile to install spike-dasm also there.

If all we need from the repo is the spike-dasm executable, perhaps I wouldn't even submodule it, but tell users to clone the riscv-isa-sim repo separately and install it there. And maybe, since the manual installation is needed only when working outside the Docker container, we could have it installed system-wide on IIS machines so users don't even have to go through these additional installation steps.

[paulsc96]

Behold my new idea: the limbo label 🥲 It labels PRs with long-term unresolved discussions. No, jokes aside, please look into this or redraft this PR.

[colluca]

I added the spike-dasm installation to the Docker container.

Last thing missing is a proper flow for installation on IIS machines. Specifically:

• settle on a set of reproducible instructions (e.g. with a specific GCC version)
• add the installation instructions to the CI (either on the snitch_cluster or riscv_isa_sim repo)
• distribute a pre-built spike-dasm binary to IIS machines or provide instructions to install it (either on the snitch_cluster repo or point to riscv_isa_sim repo)

[colluca]

I added releases to the riscv-isa-sim repo for both IIS machines and the Ubuntu-based Docker container.

Next steps involve:

• Install spike-dasm on the Docker container from the official release
• Install spike-dasm on the Gitlab CI and add installation instructions to the getting started guide