vm_core.h: move jmpbuf between tag and prev

* vm_core.h (rb_vm_tag): move jmpbuf between tag and prev so ensure to
  be accessible.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40806 b2dd03c8-39d4-4d8f-98ff-823fe69b080e