rbinstall.rb: CONFIG['arch'] not $arch

* tool/rbinstall.rb: use CONFIG['arch'] instead of $arch which was not
  added finally.  fix r39108.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39131 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/rbinstall.rb

@@ -306,7 +306,7 @@ goruby_install_name = "go" + ruby_install_name
 bindir = CONFIG["bindir", true]
 libdir = CONFIG["libdir", true]
 rubyhdrdir = CONFIG["rubyhdrdir", true]
-archhdrdir = CONFIG["rubyarchhdrdir"] || (rubyhdrdir + "/" + $arch)
+archhdrdir = CONFIG["rubyarchhdrdir"] || (rubyhdrdir + "/" + CONFIG['arch'])
 rubylibdir = CONFIG["rubylibdir", true]
 archlibdir = CONFIG["rubyarchdir", true]
 sitelibdir = CONFIG["sitelibdir"]