gdbstub: don't use target_ulong while handling registers

gdbstub: don't use target_ulong while handling registers

This is a hangover from the original code. addr is misleading as it is
only really a register id. While len will never exceed
MAX_PACKET_LENGTH I've used size_t as that is what strlen returns.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230302190846.2593720-20-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-20-richard.henderson@linaro.org>