Support of gdb for Windows 64 native systems

[Joel Brobecker]

> I propose goind the other way around.  Start by adding the needed configury
> bits needed, a mingw.mt file, and disabling the cygwin specific bits in
> win32-nat.c around #ifdef __CYGWIN__ blocks.

That's mostly how I did it, except for the win32-nat.c changes.
I like your suggestions. I think we can get something minimal
pretty quickly. Just to avoid duplicating efforts, who's in charge?
:-)

-- 
Joel