Move the nullability checks to normal clang flags

Author: gbaraldi

src/Makefile

@@ -29,7 +29,7 @@ endif
 JCFLAGS += -Wold-style-definition -Wstrict-prototypes -Wc++-compat
 
 ifeq ($(USECLANG),1)
-FLAGS += -Wno-return-type-c-linkage -Wno-atomic-alignment
+FLAGS += -Wno-return-type-c-linkage -Wno-atomic-alignment -Wno-nullability-extension -Wno-nullability-completeness # required to be allowed to use nullability extension and not be required to annotate all of everything
 endif
 
 ifeq (${USE_THIRD_PARTY_GC},mmtk)