forwardable.rb: reduce allocations in def_delegators

* lib/forwardable.rb (Forwardable::FILE_REGEXP): create regexp object
  outside sources for eval, to reduce allocations in def_delegators
  wrappers.  //o option does not make each regexps shared.  patch by
  tmm1 (Aman Gupta) in [ruby-core:53620] [Bug #8143].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39875 b2dd03c8-39d4-4d8f-98ff-823fe69b080e