timer: Added SDL_GetTicks64(), for a timer that doesn't wrap every ~49 days.

Note that this removes the timeGetTime() fallback on Windows; it is a
32-bit counter and SDL2 should never choose to use it, as it only is needed
if QueryPerformanceCounter() isn't available, and QPC is _always_ available
on Windows XP and later.

OS/2 has a similar situation, but since it isn't clear to me that similar
promises can be made about DosTmrQueryTime() even in modern times, I decided
to leave the fallback in, with some heroic measures added to try to provide a
true 64-bit tick counter despite the 49-day wraparound. That approach can
migrate to Windows too, if we discover some truly broken install that doesn't
have QPC and still depends on timeGetTime().

Fixes #4870.

src/timer/dummy/SDL_systimer.c

@@ -41,8 +41,8 @@ SDL_TicksQuit(void)
     ticks_started = SDL_FALSE;
 }
 
-Uint32
-SDL_GetTicks(void)
+Uint64
+SDL_GetTicks64(void)
 {
     if (!ticks_started) {
         SDL_TicksInit();