fpu: Remove use of int_fast16_t in conversions to int16

Make the functions which convert floating point to 16 bit integer
return int16_t rather than int_fast16_t, and correspondingly use
int_fast16_t in their internal implementations where appropriate.

(These functions are used only by the ARM target.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Message-id: 1453807806-32698-2-git-send-email-peter.maydell@linaro.org