More work towards hybrid support

* All packaging architecture dependent variables do now have a
  respective suffix and are set up for each configured packaging
  architecture, save for the kernel and boot loader variables, which
  are still only set up for the primary architecture.
  For convenience TARGET_PACKAGING_ARCH, TARGET_ARCH, TARGET_LIBSUPC++,
  and TARGET_LIBSTDC++ are set to the respective values for the primary
  packaging architecture by default.
* Introduce a set of MultiArch* rules to help with building targets for
  multiple packaging architectures. Generally the respective targets are
  (additionally) gristed with the packaging architecture. For libraries
  the additional grist is usually omitted for the primary architecture
  (e.g. libroot.so and <x86>libroot.so for x86_gcc2/x86 hybrid), so that
  Jamfiles for targets built only for the primary architecture don't
  need to be changed.
* Add multi-arch build support for all targets needed for the stage 1
  cross devel package as well as for libbe (untested).

Jamfile

@@ -4,7 +4,12 @@ NotFile doc_files ;
 Depends files : doc_files ;
 
 # Prepare the optional build features before parsing the Jamfile tree.
-include [ FDirName $(HAIKU_BUILD_RULES_DIR) BuildFeatures ] ;
+local architectureObject ;
+for architectureObject in [ MultiArchSubDirSetup ] {
+	on $(architectureObject) {
+		include [ FDirName $(HAIKU_BUILD_RULES_DIR) BuildFeatures ] ;
+	}
+}
 
 # Include packages that are required by all images.
 if ! $(HAIKU_BOOTSTRAP_BUILD) {