Clean __osMalloc.c (#4351)

* Remove #ifs

* Add `NODE_IS_VALID`

* Fix current macros

* `NODE_GET_NEXT` & `NODE_GET_PREV`

* Add macros

* Use macros and general cleanup

* Fix build (hopefully)

* Address review

soh/src/code/z_malloc.c

@@ -106,5 +106,5 @@ void ZeldaArena_Cleanup() {
 }
 
 u8 ZeldaArena_IsInitalized() {
-    return __osMallocIsInitalized(&sZeldaArena);
+    return __osMallocIsInitialized(&sZeldaArena);
 }