The attached patch updates gmp to the latest release 6.3.0.

I have tested it on x86_64 and the x86_64-w64-mingw32 cross-compilation
target, since I have taken out a work-around for this target.

There are close to 20000 dependent packages, but as far as I can see
bootstrapping is not concerned, which starts from gmp-6.0.

It would be interesting to try whether we can bootstrap directly from
6.3.0 now, that is, whether the comment
  ;; We keep this one around to bootstrap GCC, to work around a compilation
  ;; issue on ARM.  See
  ;; <https://gmplib.org/list-archives/gmp-bugs/2015-December/003848.html>.
is still valid; it might let us drop the two gmp patches altogether.

Andreas