Support of gdb for Windows 64 native systems

[Pedro Alves]

Thinking only of the target part of the story, and assuming the
debug API is similar, the x64/i386 support in GDB
could be done similarly to how x86/arm is split in gdbserver.

It would probably be much easier to start with porting gdbserver
and cross-debugging.  gdbserver already supports x86 mingw.

Take a look at win32-low.c, win32-i386-low.c and win32-arm-low.c
in gdb/gdbserver/.  On the gdb side, you would need at
least a gdb/x64-(mingw|win32)-tdep.c and/or a i386-(mingw|win32)-tdep.c.

Cheers,
Pedro Alves