gdbstub: introduce gdb_get_max_cpus

This is needed for handling vcont packets as the way of calculating
max cpus vhanges between user and softmmu mode.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20230302190846.2593720-17-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-17-richard.henderson@linaro.org>
4 files changed