New target methods for memory tagging support

New target methods for memory tagging support

This patch starts adding some of the generic pieces to accomodate memory
tagging.

We have three new target methods:

- supports_memory_tagging: Checks if the target supports memory tagging. This
  defaults to false for targets that don't support memory tagging.

- fetch_memtags: Fetches the allocation tags associated with a particular
  memory range [address, address + length).

  The default is to return 1 without returning any tags. This should only
  be called if memory tagging is supported.

- store_memtags: Stores a set of allocation tags for a particular memory
  range [address, address + length).

  The default is to return 1, which indicates an error. This should only
  be called if memory tagging is supported.

It also adds a control option for enabling/disabling memory tagging
manually: set memory-tagging on/off.

The default is "on", with GDB making its use conditional to the
architecture supporting memory tagging.

gdb/ChangeLog:

YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

	* printcmd.c (memtag): New static global.
	(show_memtag): New function.
	(_initialize_printcmd): Add set/show memory-tagging command.
	* remote.c (remote_target) <supports_memory_tagging>: New method
	override.
	<fetch_memtags>: New method override.
	<store_memtags>: New method override.
	(remote_target::supports_memory_tagging): New method.
	(remote_target::fetch_memtags): New method.
	(remote_target::store_memtags): New method.
	* target-delegates.c
	(dummy_target) <supports_memory_tagging>: New method override.
	<fetch_memtags>: New method override.
	<store_memtags>: New method override.
	(debug_target) <supports_memory_tagging>: New method override.
	<fetch_memtags>: New method override.
	<store_memtags>: New method override.
	(target_ops::supports_memory_tagging): New method.
	(target_ops::fetch_memtags): New method.
	(target_ops::store_memtags): New method.
	(dummy_target::supports_memory_tagging): New method.
	(dummy_target::fetch_memtags): New method.
	(dummy_target::store_memtags): New method.
	(debug_target::supports_memory_tagging): New method.
	(debug_target::fetch_memtags): New method.
	(debug_target::store_memtags): New method.
	* target.h (struct target_ops) <supports_memory_tagging>: New virtual
	method.
	<fetch_memtags: New virtual method.
	<store_memtags>: New virtual method.
	(target_supports_memory_tagging): Define.
	(target_fetch_memtags): Define.
	(target_store_memtags): Define.