target-arm: Move arm_rmode_to_sf to a shared location.

target-arm: Move arm_rmode_to_sf to a shared location.

This function will be needed for AArch32 ARMv8 support, so move it to
helper.c where it can be used by both targets. Also moves the code out
of line, but as it is quite a large function I don't believe this
should be a significant performance impact.

Signed-off-by: Will Newton <will.newton@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>