Make extension enum not scattered

[Timmmm]

Currently we have a scattered extension enum that we generally add entries to in the files that implement the extension, e.g. riscv_svinval.h contains:

enum clause extension = Ext_Svinval
function clause extensionEnabled(Ext_Svinval) = sys_enable_svinval()

I've found that this causes quite annoying circular dependencies, especially with functions that depend on lots of extensions (e.g. stuff around legalizing mie, mstatus, misa, etc.).

I propose making extension not scattered, and just putting it all in a big list in one file (riscv_extensions.sail)

enum extension = {
  Ext_S,
  Ext_Svinval,
  Ext_...
}

This can then be added early on in the compile order. I don't think there are any real downsides to this and it might actually be quite nice to see them all in one place.

The extensionEnabled() clauses should still be scattered.

[jordancarlin]

The only downside that I see is in relation to the new module system that @Alasdair is working on in #572. This would require all of the extensions to be defined in the enum even when that extension isn't included. I don't think that is a significant issue though because the extensionEnabled clause can still be in the extension specific files. Overall seems worthwhile solely for sake of dependencies.

[Timmmm]

Yeah I think that's fine. If Sail ever has unused code warnings it might be annoying but I guess we can just add something like Rust's #[allow(unused)].