gh-99108: Refactor _sha256 & _sha512 into _sha2. (#101924)

This merges their code. They're backed by the same single HACL* static library, having them be a single module simplifies maintenance.

This should unbreak the wasm enscripten builds that currently fail due to linking in --whole-archive mode and the HACL* library appearing twice.

Long unnoticed error fixed: _sha512.SHA384Type was doubly assigned and was actually SHA512Type. Nobody depends on those internal names.

Also rename LIBHACL_ make vars to LIBHACL_SHA2_ in preperation for other future HACL things.