x86: don't use opcode_length to identify pseudo prefixes

x86: don't use opcode_length to identify pseudo prefixes

This is in preparation of opcode_length going away as a field in the
templates. Identify pseudo prefixes by a base opcode of zero instead:
No real prefix has an opcode of zero. This at the same time allows
dropping a curious special case from i386-gen.

Since most attributes are identical for all pseudo prefixes, take the
opportunity and also template them.