* configure.ac: MJIT_HEADER_INSTALL_DIR to rubyarchhdrdir to
  support multiarch.

* Makefile.in (MJIT_HEADER_INSTALL_DIR): configured by multiarch.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62320 b2dd03c8-39d4-4d8f-98ff-823fe69b080e