* thread.c (rb_mutex_struct): reduce rb_mutex_t size by 8 bytes
  on 64bit platform. Patch by Eric Wong. [Feature #9068][ruby-core:58114]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43497 b2dd03c8-39d4-4d8f-98ff-823fe69b080e