gdbstub: only compile gdbstub twice for whole build

Now we have removed any target specific bits from the core gdbstub
code we only need to build it twice. We have to jump a few meson hoops
to manually define the CONFIG_USER_ONLY symbol but it seems to work.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230302190846.2593720-23-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-23-richard.henderson@linaro.org>