PR15688: Set pragma:vma when dwflpp uses _stp_umodule_relocate

PR15688: Set pragma:vma when dwflpp uses _stp_umodule_relocate

* dwflpp.cxx (dwflpp::emit_address): Use pragma:vma when needed, rather
  than trying to enable_task_finder.  This also leaves the opportunity
  for this generated code to be optimized out, such that vma-tracking
  may not be needed after all.
* elaborate.cxx (embeddedcode_info::visit_embeddedcode): Before enabling
  the vma-tracker, check if runtime=dyninst for PR15052.  This turns an
  ugly pass-4 failure into a nicer pass-2 message.
* session.cxx (systemtap_session::parse_cmdline): Don't force vma here
  for userspace -d modules; dwflpp will add pragma:vma as needed.
* tapsets.cxx (dwarf_atvar_expanding_visitor::visit_atvar_op): Don't
  enable_vma_tracker manually; dwflpp will add pragma:vma as needed.
* testsuite/systemtap.base/sdt_global_var.exp: Clear this setup_kfail.