configure.in: remove arch options from CXXFLAGS

* configure.in (RUBY_UNIVERSAL_ARCH): remove arch options from
  CXXFLAGS, not only CFLAGS.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47827 b2dd03c8-39d4-4d8f-98ff-823fe69b080e