* compile.c (rb_iseq_compile_node): accept NODE_IFUNC to support
  custom compilation.
* compile.c (NODE_POSTEXE): compile to
  "ONCE{ VMFrozenCore::core#set_postexe{...} }" with a new custom
  compiler `build_postexe_iseq()'.
* vm.c (m_core_set_postexe): remove parameters (passed by a block).



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42649 b2dd03c8-39d4-4d8f-98ff-823fe69b080e