rbinstall.rb: header targets

* tool/rbinstall.rb: separate header targets, comm-hdr for
  architecture independent headers, arch-hdr for architecture
  dependent headers, and hdr for both.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47722 b2dd03c8-39d4-4d8f-98ff-823fe69b080e