[PSDK][NTOS:KD64] Rename GCC_ULONG64 to ULPTR64 to self-document the fact it stores...

[PSDK][NTOS:KD64] Rename GCC_ULONG64 to ULPTR64 to self-document the fact it stores a pointer as a 64-bit quantity.
But the underlying GCC stupidity is still there (15 years later).

However, enable it only in 32-bit GCC builds, not in 64-bits nor with MSVC.
See commit b9cd3f2d9 (r25845) for some details.

GCC is indeed still incapable of casting 32-bit pointers up to 64-bits,
when static-initializing arrays (**outside** a function) without emitting
the error:

  "error: initializer element is not constant"

(which might somehow indicate it actually tries to generate executable
code for casting the pointers, instead of doing it at compile-time).

Going down the rabbit hole, other stupidities show up:

Our PVOID64 type and the related POINTER_64 (in 32-bit archs), or the
PVOID32 and POINTER_32 (in 64-bit archs), are all silently broken in
GCC builds, because the pointer size attributes __ptr64 and __ptr32,
which are originally MSVC-specific, are defined to nothing in _mingw.h.
(And similarly for the __uptr and __sptr sign-extension attributes.)

Clang and other sane ompilers has since then implemented those (enabled
with -fms-extensions), but not GCC. The closest thing that could exist
for GCC is to do:

  #define __ptr64 __attribute__((mode(DI)))

in order to get a 64-bit-sized pointer type with

  typedef void* __ptr64 PVOID64;

but even this does not work, with the error:

  "error: invalid pointer mode 'DI'"