Add an option to choose a smaller set of modules for Lean build

[Alasdair]

Add a SAIL_MODULES variable to the CMakeLists file that allows specifying a smaller set of modules, useful to constraint the Lean build to be something like RV64IM or similar, when that's all we need for some use cases. For example:

 cmake -S . -B build -DCMAKE_BUILD_TYPE=RelWithDebInfo -DSAIL_MODULES="I;M;riscv_termination;riscv_main"

Not sure if this is the best way to do this with CMake, or if we want to make it work more broadly other than just for Lean.

[github-actions]

Test Results

2 101 tests  ±0   2 101 ✅ ±0   15m 56s ⏱️ -2s
    1 suites ±0       0 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 57d539f. ± Comparison against base commit 6ee85c5.

[Timmmm]

I think either this should apply everywhere we use --all-modules, or It should be called something like LEAN_SAIL_MODULES? Otherwise LGTM.

[jordancarlin]

It would be good to add a test build of the model with a subset of the modules to CI. If that doesn’t happen in this PR, let’s open an issue so it doesn’t get forgotten.