opcodes: suppress internationalization on build helper tools

While one of the two actually having been instrumented (i386-gen.c) now
has that instrumentation dropped, there's still no point in honoring
such instrumentation in general (i.e. now for ia64-gen.c only), as that
only leads to a waste of resources.

With CFILES then being merely an alias of LIBOPCODES_CFILES, drop the
former variable altogether.