gdb: Add qIsAddressTagged packet

gdb: Add qIsAddressTagged packet

This commit adds a new packet, qIsAddressTagged, allowing GDB remote
targets to use it to query the stub if a given address is tagged.

Currently, the memory tagging address check is done via a read query,
where the contents of /proc/<PID>/smaps is read and the flags are
inspected for memory tagging-related flags that indicate the address is
in a memory tagged region.

This is not ideal, for example, for QEMU stub and other cases, such as
on bare-metal, where there is no notion of an OS file like 'smaps.'
Hence, the introduction of qIsAddressTagged packet allows checking
if an address is tagged in an agnostic way.

The is_address_tagged target hook in remote.c attempts to use the
qIsAddressTagged packet first for checking if an address is tagged and
if the stub does not support such a packet (reply is empty) it falls
back to using the current mechanism that reads the contents of
/proc/<PID>/smaps via vFile requests.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Approved-By: Luis Machado <luis.machado@arm.com>
Tested-By: Luis Machado <luis.machado@arm.com>