Support build for Portable Native Client outside of naclports tree.

* configure.in (RUBY_NACL): automatically locate pnacl-clang.
  (RUBY_PLATFORM): pnacl instead of le32-nacl.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47875 b2dd03c8-39d4-4d8f-98ff-823fe69b080e