Add ability to extend Mavis forms/extractors/tests for internal forks

[bdutro-mips]

This PR has 3 changes to make life easier for those who have to maintain internal Mavis forks:

1. The Mavis CMake config can be extended via an optional config file cmake/internal.cmake
2. The form and extractor registries can be extended by defining the following preprocessor macros:
   • INTERNAL_EXTRACTOR_REGISTRY: the class name of an additional extractor registry for non-public extractor definitions
   • INTERNAL_EXTRACTOR_REGISTRY_HEADER: the name of the header file where INTERNAL_EXTRACTOR_REGISTRY is defined
   • INTERNAL_FORM_REGISTRY: the class name of an additional form registry for non-public form definitions
   • INTERNAL_FORM_REGISTRY_HEADER: the name of the header file where INTERNAL_FORM_REGISTRY is defined
3. ExtensionManager ISA spec JSONs (e.g. riscv_isa_spec.json) now support an includes field that can be used to include additional ISA spec JSONs
   • Includes are processed after all other fields in the JSON, which means the included JSON can reference extensions defined in any parent JSON

I also added a make regress target and added the regression tests to the pre-merge checks.

[klingaard]

Curious, I assume this header/template is only included with the FormRegistry, right? In other words, in the model, this header is never seen. The reason I ask is 'cause of increased compile times which I've always battled with Mavis.

[bdutro-mips]

It's included once in FormRegistry.cpp and once in ExtractorRegistry.cpp

[klingaard]

Thank you, Thank you, Thank you for adding a proper regression and CI!!!