vm_core.h (struct rb_call_info_struct): improve packing

* vm_core.h (struct rb_call_info_struct): improve packing
  On x86-64, it goes from 112 to 104 bytes.  This results in
  ~20K of savings from just "./ruby -e exit"

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46800 b2dd03c8-39d4-4d8f-98ff-823fe69b080e