implement $(NATIVE_ARCHITECTURE) substvar for indextargets

apt-pkg/deb/debmetaindex.cc

@@ -133,6 +133,7 @@ static void GetIndexTargetsFor(char const * const Type, std::string const &URI,
 	 DefCompressionTypes = os.str();
       }
    }
+   std::string const NativeArch = _config->Find("APT::Architecture");
    bool const GzipIndex = _config->FindB("Acquire::GzipIndexes", false);
    for (std::vector<debReleaseIndexPrivate::debSectionEntry>::const_iterator E = entries.begin(); E != entries.end(); ++E)
    {
@@ -170,6 +171,10 @@ static void GetIndexTargetsFor(char const * const Type, std::string const &URI,
 		  Options.insert(std::make_pair("LANGUAGE", *L));
 	       if (tplMetaKey.find("$(ARCHITECTURE)") != std::string::npos)
 		  Options.insert(std::make_pair("ARCHITECTURE", *A));
+	       else if (tplMetaKey.find("$(NATIVE_ARCHITECTURE)") != std::string::npos)
+		  Options.insert(std::make_pair("ARCHITECTURE", NativeArch));
+	       if (tplMetaKey.find("$(NATIVE_ARCHITECTURE)") != std::string::npos)
+		  Options.insert(std::make_pair("NATIVE_ARCHITECTURE", NativeArch));
 
 	       std::string MetaKey = tplMetaKey;
 	       std::string ShortDesc = tplShortDesc;

doc/acquire-additional-files.txt

@@ -168,6 +168,9 @@ printed as-is.
   APT::Architectures (potentially modified by sources.list options),
   e.g. "amd64", "i386" or "armel" for the 'deb' type. In type 'deb-src'
   this variable has the value "source".
+* $(NATIVE_ARCHITECTURE): The architecture apt treats as the native
+  architecture for this system configured as APT::Architecture
+  defaulting to the architecture apt itself was built for.
 
 Note that while more variables might exist in the implementation, these
 are to be considered undefined and their usage strongly discouraged. If