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

fusee/fusee-primary/Makefile

@@ -119,7 +119,7 @@ all: check_rebootstub $(BUILD)
 check_rebootstub:
 	@$(MAKE) -C $(AMS)/exosphere/rebootstub all
 
-$(BUILD):
+$(BUILD): check_rebootstub
 	@[ -d $@ ] || mkdir -p $@
 	@$(MAKE) --no-print-directory -C $(BUILD) -f $(CURDIR)/Makefile