vm_core.h: rb_thread_struct::machine

* vm_core.h (rb_thread_struct): aggregate cpu stuff into a struct,
  so that a debugger can show its content at once.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44722 b2dd03c8-39d4-4d8f-98ff-823fe69b080e