Switch atmosphere's build target to C++20. (#952)

* ams: update to build with gcc10/c++20

* remove mno-outline-atomics

* ams: take care of most TODO C++20s

* fusee/sept: update for gcc10

* whoosh, your code now uses pre-compiled headers

* make: dependency fixes

stratosphere/boot2/source/boot2_main.cpp

@@ -102,5 +102,7 @@ int main(int argc, char **argv)
 
     /* Launch all programs off of SYSTEM/the SD. */
     boot2::LaunchPostSdCardBootPrograms();
+
+    return 0;
 }