sim: cr16: fix build on gcc-12 (NULL comparison)

sim: cr16: fix build on gcc-12 (NULL comparison)

On gcc-12 build fails as:

    sim/cr16/interp.c: In function 'lookup_hash':
    sim/cr16/interp.c:89:25: error:
      the comparison will always evaluate as 'true'
      for the address of 'mnimonic' will never be NULL [-Werror=address]
       89 |   if ((h->ops->mnimonic != NULL) &&
          |                         ^~

'mnimonic' is a sharr array within ops. It can never be NULL.

While at it renamed 'mnimonic' to 'mnemonic'.