Makefile: add a 'natruntop' target, similar to 'runtop'

Makefile

@@ -343,6 +343,7 @@ partialclean::
 	rm -f ocaml
 
 RUNTOP=./byterun/ocamlrun ./ocaml -nostdlib -I stdlib -noinit $(TOPFLAGS)
+NATRUNTOP=./ocamlnat -nostdlib -I stdlib -noinit $(TOPFLAGS)
 
 runtop:
 	$(MAKE) runtime
@@ -350,6 +351,13 @@ runtop:
 	$(MAKE) ocaml
 	@rlwrap --help 2>/dev/null && rlwrap $(RUNTOP) || $(RUNTOP)
 
+natruntop:
+	$(MAKE) runtime
+	$(MAKE) coreall
+	$(MAKE) opt.opt
+	$(MAKE) ocamlnat
+	@rlwrap --help 2>/dev/null && rlwrap $(NATRUNTOP) || $(NATRUNTOP)
+
 # The native toplevel
 
 compilerlibs/ocamlopttoplevel.cmxa: $(OPTTOPLEVEL:.cmo=.cmx)