gdbstub: move register helpers into standalone include

gdbstub: move register helpers into standalone include

These inline helpers are all used by target specific code so move them
out of the general header so we don't needlessly pollute the rest of
the API with target specific stuff.

Note we have to include cpu.h in semihosting as it was relying on a
side effect before.

Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>
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: <20230302190846.2593720-21-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-21-richard.henderson@linaro.org>