make all threads preemptive. workaround file based emummc hanging due to fs r/w hanging the system.

the fix for file based emummc is to simply sleep between fs r/w to the sd card.
the performance impact is minimal, even with the reduced buffer size.

the above *only* applies for when using file based emummc. not affecting using partition or sysmmc.

sphaira/include/ui/progress_box.hpp

@@ -18,7 +18,7 @@ struct ProgressBox final : Widget {
         const std::string& action,
         const std::string& title,
         ProgressBoxCallback callback, ProgressBoxDoneCallback done = [](Result rc){},
-        int cpuid = 1, int prio = 0x2C, int stack_size = 1024*128
+        int cpuid = 1, int prio = PRIO_PREEMPTIVE, int stack_size = 1024*128
     );
     ~ProgressBox();