Parameterize AVX support

[iccuevas]

[Issue #16]
I've parameterize the AVX support using #ifdef AVX_SUPPORT. But I need some help updating the Makefile in order to avoid compiling the Jasmin AVX files
To enable AVX support, the C program has to be compiled using -DAVX_SUPPORT (we can also change the name)

[iccuevas]

I believe all AVX dependencies are now completely splitted by AVX_SUPPORT flag. To enable AVX support, the code has to be compiled using DAVX_SUPPORT C flag.

make run-ref won't compile code/files with AVX support

[sarranz]

This looks great, thanks! I have a question: do you know if compiling the dependencies with -march=native -mavx2 can cause trouble? We do this in https://github.com/formosa-crypto/jasmin-tutorial/blob/98fba54cb8da1022193cd43175583e07df35cf2e/flake.nix#L25C1-L25C53

[sarranz]

I would just wrap the whole thing inside the ifdef and not mention TESTTYPE at all if there is only one option. i.e., when AVX2 is disabled. Do you think this is a good idea? Also, it would be nice to add a sentence that says "run all the tests for gimli" or something similar.