_hmac module bypasses --with-builtin-hashlib-hashes by embedding all HACL hash implementations

Bug report

Bug description:

When configuring Python with e.g. --with-builtin-hashlib-hashes=blake2 to build only the blake2 builtin hash module, the _hmac module is still unconditionally built and embeds all HACL hash implementations (MD5, SHA1, SHA2, SHA3, Blake2).

While in these cases the excluded hashes are not built as extension modules, since they are compiled into _hmac, then md5 and other disabled digests can still be computed

>>> import _hmac
>>> _hmac.compute_digest(b'key', b'msg', 'md5')
b'\x18\xe3T\x8cY\xad@\xdd\x03\x90{z\xee\xe7\x1dg'

Not sure if that is intended behaviour or an oversight but it seems to defeat the purpose of excluding specific hashes. Also increases the size of _hmac substantially. Discovered it when working on a FIPS build.

On our FIPS build I opted for completely disabling _hmac in this case (FIPS builds only utilize OpenSSL anyway for hmac)

CPython versions tested on:

3.15, 3.14

Operating systems tested on:

Linux

Start with the linked configure.ac section and the HACL source entries in Makefile.pre.in. Configure CPython with --with-builtin-hashlib-hashes=blake2, build it, and reproduce the _hmac.compute_digest(..., 'md5') behavior. Done means the selected hash configuration is respected by _hmac and the excluded implementations are no longer embedded.