Stable-ize the way to interact with riscv-isa-sim

[eopXD]

Discussions have came up in multiple places ([1], [2]) that we should stable-ize how we should find a fixed place for linking to riscv-isa-sim, to both containing the refactoring from upstream and downstream SiFive custom instruction headers (under riscv/insns). This issue wants to track this so we don't forget this in the future.

[1] https://github.com/sifive/rvv-intrinsic-fuzzing/pull/93/files#r866516767
[2] https://github.com/sifive/rvv-intrinsic-fuzzing/pull/77

We can clone Spike only and not the whole federation repo using sparse checkout (Looks like a new feature since 2012)

https://stackoverflow.com/questions/600079/how-do-i-clone-a-subdirectory-only-of-a-git-repository
https://stackoverflow.com/questions/6238590/set-git-submodule-to-shallow-clone-sparse-checkout