efi/libstub: Measure into CC protocol if TCG2 protocol is absent

To accommodate confidential compute VMs that expose the simplified CC
measurement protocol instead of the full-blown TCG2 one, fall back to
the former if the latter does not exist.

The CC protocol was designed to be used in this manner, which is why the
types and prototypes have been kept the same where possible. So reuse
the existing code, and only deviate from the TCG2 code path where
needed.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>