Reduce the number of ifdefs within cpu_loop().

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220107213243.212806-10-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>