target/arm: Move constant expanders to translate.h

Some of the constant expanders defined in translate.c are generically
useful and will be used by the separate C files for VFP and Neon once
they are created; move the expander definitions to translate.h.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210430132740.10391-2-peter.maydell@linaro.org