The feature test functions isar_feature_*() now take up nearly
a thousand lines in target/arm/cpu.h. This header file is included
by a lot of source files, most of which don't need these functions.
Move the feature test functions to their own header file.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20231024163510.2972081-2-peter.maydell@linaro.org