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

exosphere/Makefile

@@ -156,6 +156,8 @@ $(OUTPUT).bin	:	$(OUTPUT).elf
 
 $(OUTPUT).elf	:	$(OFILES)
 
+my_libc.o: CFLAGS += -fno-builtin
+
 %.elf:
 	@echo linking $(notdir $@)
 	$(LD) $(LDFLAGS) $(OFILES) $(LIBPATHS) $(LIBS) -o $@