ARM: Ignore top 16 bits when doing VCVT from 16 bit fixed point

ARM: Ignore top 16 bits when doing VCVT from 16 bit fixed point

VCVT of 16 bit fixed point to float should ignore the top 16 bits
of the source register. Cast to int16_t and friends rather than
int16 -- the former is guaranteed exactly 16 bits wide where the
latter is merely at least 16 bits wide (and so is usually 32 bits).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Nathan Froyd <froydnj@codesourcery.com>