x86: add generated tables dependency check to gas

x86: add generated tables dependency check to gas

As requested by H.J., just for the sake of people potentially building
in gas/ alone, add a check that the generated files in opcodes/ are
actually up-to-date. Personally I think this should at best be a
warning, but I can see how this may not be easily noticable among other
make output (depending in particular on the verbosity level).