symm/gcm.c: Make `gcm_mktable' and `gcm_mulk_...' be CPU-dependent.

A couple of other changes to ease the way:

  * Split `gcm_mulk_...' into two endianness variants, so that
    CPU-specific variants don't have to track what's going on through
    the key table.

  * Abstract out `recover_k' to decode the key value from a table, for
    the use of `gcm_concat'.  This is, of course, necessary if the table
    format is CPU-dependent.

  * Add testing to make sure that `mktable'/`recover_k' agree with each
    other.

There are currently no fancy implementations, but you can tell what's
coming.  No actual functional change, except for logging if you set
`CATACOMB_CPUDISPATCH_DEBUG' in the environment.