Ruby

regexec.c: fix build with direct threaded VM

• regexec.c (match_at): move direct threaded VM code to get rid of
  mixed declarations and code, and enable it only for gcc since it
  depends on a gcc extension.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53251 b2dd03c8-39d4-4d8f-98ff-823fe69b080e