Experimental 64-bit windows GP binary

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

Dear PARI developers,

Following a contribution of Eric Driver, we have managed
to build an experimental 64-bit windows binary that pass all the tests.
It is available at:
<http://pari.math.u-bordeaux.fr/pub/pari/windows/snapshots/gp-mingw64-gfa553d7>
It includes GNU GMP and readline support.
Please test! (and see how much it is faster than the 32-bit version).

I like to thanks Eric for starting this project and doing most of the work.

Hopefully, we will be able to generate daily 64-bit windows binaries in the
future.

The code is available in the GIT branch bill-win64, but you also need
to apply Eric script:
<http://pari.math.u-bordeaux.fr/pub/pari/windows/snapshots/convertMingw642.sh>

Cheers,
Bill