gdbstub: Add support for MTE in system mode

gdbstub: Add support for MTE in system mode

This commit makes handle_q_memtag, handle_q_isaddresstagged, and
handle_Q_memtag stubs build for system mode, allowing all GDB
'memory-tag' subcommands to work with QEMU gdbstub on aarch64 system
mode.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/620
Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240906143316.657436-3-gustavo.romero@linaro.org>
[AJB: add #ifdef CONFIG_TCG guards]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240910173900.4154726-8-alex.bennee@linaro.org>