gdbstub: move GDBState to shared internals header

We are about to split softmmu and user mode helpers into different
files. To facilitate this we will need to share access to the GDBState
between those files.

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

Message-Id: <20230302190846.2593720-7-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-7-richard.henderson@linaro.org>