Removed SDL_ATOMIC_DISABLED

It turns out that because we redefine SDL functions internally, it is safe to call SDL mutex functions while initializing the jump table

CMakeLists.txt

@@ -210,7 +210,6 @@ if(EMSCRIPTEN)
 
   set(SDL_ASSEMBLY_DEFAULT OFF)
   set(SDL_SHARED_AVAILABLE OFF)
-  set(SDL_ATOMIC_DEFAULT OFF)
 endif()
 
 if(VITA OR PSP OR PS2 OR N3DS OR RISCOS)
@@ -238,7 +237,6 @@ if(SDL_SHARED_DEFAULT AND SDL_STATIC_DEFAULT AND SDL_SHARED_AVAILABLE)
 endif()
 
 set(SDL_SUBSYSTEMS
-  Atomic
   Audio
   Video
   Render