Make the VFP_CONV_FIX helpers a little more flexible in
preparation for the A64 uses. This requires two changes:
 * use the correct softfloat conversion function based on itype
   rather than always the int32 one; this is possible now that
   softfloat provides int16 versions and necessary for the
   future conversion-to-int64 A64 variants. This also allows
   us to drop the awkward 'sign' macro argument.
 * split the 'fsz' argument which currently controls both
   width of the input float type and width of the output
   integer type into two; this will allow us to specify the
   A64 64-bit-int-to-single conversion function, where the
   two widths are different.

We can also drop the (itype##_t) cast now that softfloat
guarantees that all the itype##_to_float* functions take
an integer argument of exactly the correct type.

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