vm_core.h (struct rb_iseq_struct): reduce to 288 bytes on 64-bit

* vm_core.h (struct rb_iseq_struct): reduce to 288 bytes on 64-bit
  (found with pahole, from the dwarves package)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46814 b2dd03c8-39d4-4d8f-98ff-823fe69b080e