Makefile: the promote-cross target should use cp rather than stripdebug

The compiler's bootstrap procedure involves calling the promote-cross
target of the root Makefile.

When this target is invoked e.g. in the context of an executable
magic number update, it is actually not possible to use the stripdebug
tool.

Thus, this commit makes sure the promote-cross target uses cp
to promote the bytecode tools, rather than stripdebug.

This is not an issue because in the promote rule used later
during the bootstrap the stripdebug tool can be used, so that the
bytecode executables one gets after a complete bootstrap will remain
small and won't contain any debugging symbol.

Makefile

@@ -434,19 +434,27 @@ compare:
 	else echo "Fixpoint not reached, try one more bootstrapping cycle."; \
 	fi
 
+# Promote a compiler
+
+PROMOTE ?= cp
+
+.PHONY: promote-common
+promote-common:
+	$(PROMOTE) ocamlc boot/ocamlc
+	$(PROMOTE) lex/ocamllex boot/ocamllex
+	cp yacc/ocamlyacc$(EXE) boot/ocamlyacc$(EXE)
+	cd stdlib; cp $(LIBFILES) ../boot
+
 # Promote the newly compiled system to the rank of cross compiler
 # (Runs on the old runtime, produces code for the new runtime)
 .PHONY: promote-cross
-promote-cross:
-	$(CAMLRUN) tools/stripdebug ocamlc boot/ocamlc
-	$(CAMLRUN) tools/stripdebug lex/ocamllex boot/ocamllex
-	cp yacc/ocamlyacc$(EXE) boot/ocamlyacc$(EXE)
-	cd stdlib; cp $(LIBFILES) ../boot
+promote-cross: promote-common
 
 # Promote the newly compiled system to the rank of bootstrap compiler
 # (Runs on the new runtime, produces code for the new runtime)
 .PHONY: promote
-promote: promote-cross
+promote: PROMOTE = $(CAMLRUN) tools/stripdebug
+promote: promote-common
 	cp byterun/ocamlrun$(EXE) boot/ocamlrun$(EXE)
 
 # Remove old bootstrap compilers