update to sail simulator build instructions and plugin, necessary for newer version of sail

[jeras]

The instructions for building sail did not work properly.
I updated the instructions and the names of the executables created by the newer version of sail.

[Alasdair]

Looks good to me, the version of OCaml listed there is 7 years old at this point so I suspect those instructions haven't worked in quite a while.

We only very recently switched (within the last month or so) to using cmake in the sail-riscv repo, so there may be some teething problems with the switch, please let us know over there if you run into further issues!

[jordancarlin]

It probably makes sense to direct most users to install the sail compiler using the precompiled binary instead of making everyone deal with opam. See the releases page of the Sail repo: https://github.com/rems-project/sail/releases.

[jeras]

I agree with @jordancarlin, regarding using the binary, it should be the recommended option in the RISCOF instructions. I only checked how NEORV32 is handling it and a binary is used there. Installing another package manager on your PC is a lot of storage and maintenance overhead.

The current instructions could probably replace the use of docker images with using a binary. I suspect the docker image is also an antique.

While following instructions I also encountered some other issues:

• typos in generated plugin code,
• if spike is used for reference, the generated plugin is incomplete, the CLI for spike only contains the executable, it is missing all arguments.