gdbstub: make various helpers visible to the rest of the module

We will be needing to use these helpers between the user and softmmu
files so declare them in the headers, add a system prefix and remove
static from the implementations.

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

Message-Id: <20230302190846.2593720-10-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-10-richard.henderson@linaro.org>