x86: change fetch error handling for get<N>()

x86: change fetch error handling for get<N>()

Make them return boolean and convert FETCH_DATA() uses to fetch_code().
With this no further users of FETCH_DATA() remain, so the macro and its
backing function are dropped as well.

Leave value types as they were for the helper functions, even if I don't
think that beyond get64() use of bfd_{,signed_}vma is really necessary.
With type change of "disp" in OP_E_memory(), change the 2nd parameter of
print_displacement() to a signed type as well, though (eliminating the
need for a local variable of signed type). This also eliminates the need
for custom printing of '-' in Intel syntax displacement expressions.

While there drop forward declarations which aren't really needed.