configure.in: build probes with systemtap's dtrace wrapper

* configure.in (RUBY_DTRACE_POSTPROCESS): Fix compatibility with
  systemtap on linux. stap requires `dtrace -G` post-processing, but
  the dtrace compatibility wrapper is very strict about probes.d
  syntax.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44254 b2dd03c8-39d4-4d8f-98ff-823fe69b080e