libstrat: enable -Wextra, -Werror

This caught an embarrassingly large number of bugs.

libraries/libstratosphere/source/fssystem/fssystem_thread_priority_changer.cpp

@@ -19,6 +19,7 @@ namespace ams::fssystem {
 
     s32 ScopedThreadPriorityChangerByAccessPriority::GetThreadPriorityByAccessPriority(AccessMode mode) {
         /* TODO: Actually implement this for real. */
+        AMS_UNUSED(mode);
         return os::GetThreadPriority(os::GetCurrentThread());
     }