Add GDB-side remote target support for memory tagging

Add GDB-side remote target support for memory tagging

This patch adds memory tagging support to GDB's remote side, with
packet string checks, new packet support and an implementation of
the two new tags methods fetch_atags and store_atags.

gdb/ChangeLog:

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

	* remote.c (PACKET_memory_tagging_feature): New enum.
	(remote_memory_tagging_p): New function.
	(remote_protocol_features): New "memory-tagging" entry.
	(remote_target::remote_query_supported): Handle memory tagging
	support.
	(remote_target::supports_memory_tagging): Implement.
	(create_fmemtags_request, parse_fmemtags_reply)
	(create_smemtags_request): New functions.
	(remote_target::fetch_memtags): Implement.
	(remote_target::store_memtags): Implement.
	(_initialize_remote): Add new "memory-tagging-feature"
	config command.