Expand gemspec path to the real path

`make install` has loaded forwardable.rb twice, from
forwardable.gemspec and prime.gemspec.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64882 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/rbinstall.rb

@@ -726,6 +726,7 @@ install?(:ext, :arch, :gem, :'default-gems', :'default-gems-arch') do
 end
 
 def load_gemspec(file)
+  file = File.realpath(file)
   code = File.read(file, encoding: "utf-8:-")
   code.gsub!(/`git.*?`/m, '""')
   begin