because it's not supported by this file. Also, shared `def_iseq_ptr`
instead of copying the main definition of it.

vm_core.h: moved `def_iseq_ptr` to this place. added `inline` to avoid
compiler warnings since it's not used in some files including vm_core.h.
vm_insnhelper.c: moved `def_iseq_ptr` to vm_core.h.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65440 b2dd03c8-39d4-4d8f-98ff-823fe69b080e