(merged partially from r42927)

* lib/rubygems/core_ext/kernel_require.rb: Backport a fix for concurrent
  requires. [ruby-core:58918] [Backport #9224]
  see also https://github.com/rubygems/rubygems/pull/833


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@45100 b2dd03c8-39d4-4d8f-98ff-823fe69b080e