target_ops::to_stratum -> target_ops::stratum() virtual method

Given that a target's stratum is a property of the type, and not of an
instance of the type, get rid of to_stratum data field and replace it
with a virtual method.

I.e., when we have e.g., 10 target remote instances active, there's no
need for each of the instances to have their own to_stratum copy.

gdb/ChangeLog:
2018-11-30  Pedro Alves  <palves@redhat.com>

	* aix-thread.c (aix_thread_target) <aix_thread_target>: Delete.
	<stratum>: New override.
	* bfd-target.c (aix_thread_target) <aix_thread_target>: Delete.
	<stratum>: New override.
	* bsd-uthread.c (bsd_uthread_target) <bsd_uthread_target>: Delete.
	<stratum>: New override.
	* exec.c (exec_target) <exec_target>: Delete.
	<stratum>: New override.
	* gdbarch-selftests.c (register_to_value_test): Adjust to use the
	stratum method instead of the to_stratum field.
	* linux-thread-db.c (thread_db_target) <thread_db_target>: Delete.
	<stratum>: New override.
	(thread_db_target::thread_db_target): Delete.
	* make-target-delegates (print_class): Don't print a ctor
	declaration.  Print a stratum method override declaration.
	* process-stratum-target.h (process_stratum_target)
	<process_stratum_target>: Delete.
	<stratum>: New override.
	* ravenscar-thread.c (ravenscar_thread_target)
	<ravenscar_thread_target>: Delete.
	<stratum>: New override.
	* record-btrace.c (record_btrace_target)
	<record_btrace_target>: Delete.
	<stratum>: New override.
	* record-full.c (record_full_base_target)
	<record_full_base_target>: Delete.
	<stratum>: New override.
	* record.c (record_disconnect, record_detach)
	(record_mourn_inferior, record_kill): Adjust to use the stratum
	method instead of the to_stratum field.
	* regcache.c (cooked_read_test, cooked_write_test): Likewise.
	* sol-thread.c (sol_thread_target)
	<sol_thread_target>: Delete.
	<stratum>: New override.
	* spu-multiarch.c (spu_multiarch_target)
	<spu_multiarch_target>: Delete.
	<stratum>: New override.
	* target-delegates.c: Regenerate.
	* target.c (target_stack::push, target_stack::unpush)
	(pop_all_targets_above, pop_all_targets_at_and_above)
	(info_target_command, target_require_runnable)
	(target_stack::find_beneath): Adjust to use the stratum method
	instead of the to_stratum field.
	(dummy_target::dummy_target): Delete.
	(dummy_target::stratum): New.
	(debug_target::debug_target): Delete.
	(debug_target::stratum): New.
	(maintenance_print_target_stack): Adjust to use the stratum method
	instead of the to_stratum field.
	* target.h (struct target_ops) <stratum>: New method.
	<to_stratum>: Delete.
	<is_pushed>: Adjust to use the stratum method
	instead of the to_stratum field.

gdb/sol-thread.c

@@ -78,12 +78,11 @@ static const target_info thread_db_target_info = {
 class sol_thread_target final : public target_ops
 {
 public:
-  sol_thread_target ()
-  { this->to_stratum = thread_stratum; }
-
   const target_info &info () const override
   { return thread_db_target_info; }
 
+  strata stratum () const override { return thread_stratum; }
+
   void detach (inferior *, int) override;
   ptid_t wait (ptid_t, struct target_waitstatus *, int) override;
   void resume (ptid_t, int, enum gdb_signal) override;