a0 vs a7 for system calls

[TheThirdOne]

Jupiter uses a0 for the system call number. Most other simulators/kernels use a7 for the system call number. Notably, spike with pk, rv8, qemu for userspace, RARS, and RIPES all use a7. Venus would be the notable simulator that uses a0.

Would you be interested in changing to use a7 instead of a0? I would be willing to make a PR, but wanted to get a "go ahead" before I start.

[andrescv]

Yes absolutely, in the beginning of the project I thought that register a0 would be a good choice for system calls because we used to use Venus in classroom. If you could make a PR with that change, go ahead totally agree with the change.