Do not update gems when building with base ruby

diff -Naur ruby.old/defs/gmake.mk ruby.new/defs/gmake.mk
--- ruby.old/common.mk
+++ ruby.new/common.mk
@@ -1298,7 +1298,7 @@ update-config_files: PHONY
 	    config.guess config.sub
 
 refresh-gems: update-bundled_gems prepare-gems
-prepare-gems: $(HAVE_BASERUBY:yes=update-gems) $(HAVE_BASERUBY:yes=extract-gems)
+prepare-gems:
 
 update-gems$(gnumake:yes=-nongnumake): PHONY
 	$(ECHO) Downloading bundled gem files...