Check that native-code is supported in world.opt

Previously, if the native-code compiler is not supported
(e.g. because configure doesn't recognize the target architecture,
or is run with --disable-native-compiler), "make world.opt"
fails late and with a mysterious error message
(a warning about undeclared C functions in runtime/roots_nat.c
or runtime/backtrace_nat.c, typically).

This commit ensures that "make world.opt", "make opt" and "make opt.opt"
fail immediately with a meaningful error message if the native-code
compiler is not supported.

Note: in "world.opt", make sure that "checknative" is run, and only
then "coldstart"; don't run them in parallel.

Makefile

@@ -401,7 +401,7 @@ opt-core: runtimeopt
 	$(MAKE) libraryopt
 
 .PHONY: opt
-opt:
+opt: checknative
 	$(MAKE) runtimeopt
 	$(MAKE) ocamlopt
 	$(MAKE) libraryopt
@@ -409,7 +409,7 @@ opt:
 
 # Native-code versions of the tools
 .PHONY: opt.opt
-opt.opt:
+opt.opt: checknative
 	$(MAKE) checkstack
 	$(MAKE) runtime
 	$(MAKE) core
@@ -464,7 +464,8 @@ world: coldstart
 
 # Compile also native code compiler and libraries, fast
 .PHONY: world.opt
-world.opt: coldstart
+world.opt: checknative
+	$(MAKE) coldstart
 	$(MAKE) opt.opt
 
 # FlexDLL sources missing error messages
@@ -1130,6 +1131,16 @@ ocamldebugger: ocamlc ocamlyacc ocamllex otherlibraries
 partialclean::
 	$(MAKE) -C debugger clean
 
+# Check that the native-code compiler is supported
+.PHONY: checknative
+checknative:
+ifeq "$(ARCH)" "none"
+checknative:
+	$(error The native-code compiler is not supported on this platform)
+else
+	@
+endif
+
 # Check that the stack limit is reasonable (Unix-only)
 .PHONY: checkstack
 checkstack: