target/arm: Add missing space in comment

Fix a missing space before a comment terminator.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220819110052.2942289-7-peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>