Fix hax extraction of crates depending on some libcrux crates.

[maximebuyse]

When extracting a crate with hax that depends on some libcrux crates, we need these libcrux crates to compile under the hax cfg. This can be prevented by:

• Some items being marked cfg(not(hax)) (using hax_lib::exclude is better to remove the problematic items for verification of libcrux using hax).
• The use of libcrux-intrinsics instead of driect use of intrinsics from core_arch needs to be consistent across different libcrux crates.

[maximebuyse]

There is still a problem with the use of transmute between u128 and __m128i under the hax feature, but this doesn't block extraction.

[maximebuyse]

This breaks the hax extraction of ml-kem because of this macro. We need the impl produced by the macro to be excluded from the extraction, but adding hax_lib::exclude inside the macro means other crates using this macro need to have hax-lib as a dependency, which is not the case now. Maybe we could add this attribute conditionnally.

[franziskuskiefer]

@maximebuyse what's the state here? We should try to get this resolved to allow extraction of dependent crates.

[maximebuyse]

@maximebuyse what's the state here? We should try to get this resolved to allow extraction of dependent crates.

I think I found a way to make it work! I opened a new PR (#1312), I don't want to break things for freedomofpress/securedrop-protocol#131 by messing up this branch.