Provide kernels with true reference implementations for quantized ops (#4108)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4108

We want to be able to run the reference implementations on x86, so we don't want any intrinsics or anything like that in the reference kernels.

In the end, this change has a lot of things:
- introduce a `reference` folder for reference implementations
- moved the primary cmake flow from HiFi to reference, so that the default mode can run on x86
- that means we will need a proper flag to use HiFi optimized ops, which we will add later
- add a `quantized_matmul` reference kernel

Reviewed By: dulinriley

Differential Revision: D59238748

fbshipit-source-id: 830c89fe9ee8dd87ece963e1174ca3cbd1e0fbc6