[amd64-windows] Fix function calls on amd64-windows

[Joel Brobecker]

Hello,

The following patch series enhances the amd64 function-call and
return-value code to handle the Microsoft ABI for amd64-windows.
As it turned out, the principles where relatively similar to
the ABI used on Linux systems, but the details where quite significantly
different (list of integer registers, parameters passed by memory,
etc).

These patches were tested on x86_64-linux using the official testsuite,
as well as on x86_64-windows, but using AdaCore's testsuite.

I'd like to commit in a few days if there are no objection.