intrinsic-test: bringing back support for --generate-only flag

[madhav-madhusoodanan]

Summary

1. Added support for the case when the --generate-only flag is passed. This helps with generating the Rust and C artifacts even if the compilation environment is not supported

Context

This PR is part of the changes that were originally made in the x86 extension PR #1814 for intrinsic-test, and is intended to accomodate the changes made in PRs #1862 and #1863.

cc: @folkertdev @Amanieu

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[folkertdev]

r? folkertdev

[folkertdev]

Nice, I think we can do better (using some custom types), I'll look into that, but this at least unbreaks the workflow.

[madhav-madhusoodanan]

Yes, that was my intention too.
A common interface for the common module so that we can both work in parallel, unblocked with the changes in each other's workflows.