Use libstdc++, libsupc++ and libgcc from gcc_syslibs.

* Instead of faking libstdc++.so from libstdc++.a, use libstdc++.so
  from the gcc_syslibs build feature for everything except x86_gcc2.
* Use libgcc_s.so from the gcc_syslibs build feature for everything but
  x86_gcc2 (which still carries libgcc as part of libroot.so).
* Drop filtering of libgcc objects for libroot, as that is no longer
  necessary since we're only using libgcc-as-single-object for libroot
  with x86_gcc2, where the filtered object file doesn't exist. Should
  the objects that used to be filtered cause any problems as part of
  libgcc_s.so, we can always filter them as part of the gcc build.
* Use libsupc++.so from the gcc_syslibs build feature for everything but
  x86_gcc2.
* Adjust all Jamfiles accordingly.
* Deactivate building of faked libstdc++.so for non-x86-gcc2. For
  x86_gcc2, we still build libstdc++.so from the sources in the Haiku
  source tree as part of the Haiku build .
* Put gcc_syslibs package onto the image, when needed.

build/jam/BootRules

@@ -54,9 +54,11 @@ rule BootLd
 	# libsupc++ is opt-out.
 	local libs ;
 	if ! [ on $(1) return $(HAIKU_NO_LIBSUPC++) ] {
-		libs += $(TARGET_BOOT_LIBSUPC++) ;
+		libs += [ TargetBootLibsupc++ true ] ;
+		Depends $(1) : [ TargetBootLibsupc++ ] ;
 	}
-	LINKLIBS on $(1) =  $(libs) $(TARGET_BOOT_LIBGCC) ;
+	LINKLIBS on $(1) =  $(libs) [ TargetBootLibgcc true ] ;
+	Depends $(1) : [ TargetBootLibgcc ] ;
 
 	# TODO: Do we really want to invoke SetupBoot here? The objects should
 	# have been compiled with BootObjects anyway, so we're doing that twice.