mkmf.rb: fix up r56053

* lib/mkmf.rb (timestamp_file): TARGET_SO_DIR is defaulted to
  RUBYARCHDIR but different for each extension libraries.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56054 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -2019,7 +2019,7 @@ preload = #{defined?($preload) && $preload ? $preload.join(' ') : ''}
   end
 
   def timestamp_file(name, target_prefix = nil)
-    pat = {}
+    pat = {'TARGET_SO_DIR'=>'$(RUBYARCHDIR)'}
     install_dirs.each do |n, d|
       pat[n] = $` if /\$\(target_prefix\)\z/ =~ d
     end