to rb_execution_context_t.

* vm_core.h (rb_thread_t): move
  `rb_thread_t::interrupt_flag` and
  `rb_thread_t::interrupt_mask` to rb_execution_context_t.

  RUBY_VM_CHECK_INTS() accepts `ec` instead of `th`.

* cont.c (rb_fiber_terminate): to propagate interrupt information,
  add new parameter `need_interrupt`.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60672 b2dd03c8-39d4-4d8f-98ff-823fe69b080e