PR10943: support @entry saving in .return probes

PR10943: support @entry saving in .return probes

* staptree.h (struct entry_op): New.
* parse.cxx (parse_entry_op): Parse @entry into an entry_op.
* elaborate.cxx, staptree.cxx, translate.cxx: Add entry_op boilerplate.
* tapsets.cxx (dwarf_var_expanding_visitor::visit_entry_op): New, save
  operand value using gen_mapped_saved_return.
  (dwarf_var_expanding_visitor::gen_mapped_saved_return): Use a new flag
  to see if the tid is already saved, because the add_block may have
  been created by gen_kretprobe_saved_return instead.
* NEWS, stapprobes.3stap: Document it.
* testsuite/sem*/entry*.stp: Test it.