target/arm: hide aliased MIDR from gdbstub

This is just a constant alias register with the same value as the
"other" MIDR so it serves no purpose being presented to gdbstub.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20231106185112.2755262-6-alex.bennee@linaro.org>