Fix GC_CreateThread 'dwStackSize' argument type for Win64

Fix GC_CreateThread 'dwStackSize' argument type for Win64

* include/gc.h (GC_uintptr_t): Define (for Win32/CE) even if
GC_NO_THREAD_DECLS; move definition upper than GC_CreateThread.
* include/gc.h (GC_WIN32_SIZE_T): New macro (defined as GC_uintptr_t,
Win32 threads only).
* include/gc.h (GC_CreateThread): Use GC_WIN32_SIZE_T instead of DWORD
type for "dwStackSize" argument (Win32 threads only).
* win32_threads.c (GC_CreateThread): Likewise.
* tests/test.c (fork_a_thread): Verify types of GC_CreateThread
arguments by explicit casts (only if GC_WIN32_THREADS).