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/dynapi/SDL_dynapi_overrides.h

@@ -823,3 +823,4 @@
 #define SDL_asprintf SDL_asprintf_REAL
 #define SDL_vasprintf SDL_vasprintf_REAL
 #define SDL_GetWindowICCProfile SDL_GetWindowICCProfile_REAL
+#define SDL_GetTicks64 SDL_GetTicks64_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -890,3 +890,4 @@ SDL_DYNAPI_PROC(int,SDL_asprintf,(char **a, SDL_PRINTF_FORMAT_STRING const char
 #endif
 SDL_DYNAPI_PROC(int,SDL_vasprintf,(char **a, const char *b, va_list c),(a,b,c),return)
 SDL_DYNAPI_PROC(void*,SDL_GetWindowICCProfile,(SDL_Window *a, size_t *b),(a,b),return)
+SDL_DYNAPI_PROC(Uint64,SDL_GetTicks64,(void),(),return)