Fixed PR 10258.

Fixed PR 10258.
* tapset-procfs.cxx (procfs_derived_probe::join_group): Outputs structure
  definition.
  (procfs_derived_probe_group::emit_module_decls): The generated
  '_stp_procfs_read' and '_stp_procfs_write' functions no longer put a
  string_t on the stack.
  (procfs_var_expanding_visitor::visit_target_symbol): The generated
  '_procfs_value_set' and '_procfs_value_get' functions copy directly
  into or out of the os buffer.