util: add fp64 -> fp32 conversion support for RTNE and RTZ rounding modes

util: add fp64 -> fp32 conversion support for RTNE and RTZ rounding modes

In order to be coherent with the pre-existent API for half floats,
this new API for double is the one meant to be used when doing double
to float conversions. It is no more than a wrapper for the softfloat.h
API but we meant to keep that one private.

v2:
- Fix bug in _mesa_double_to_float_rtz() in the inf/nan detection
  using the exponent value.

v3:
- Replace custom f64 -> f32 implementations with the softfloat
  one (Andres).

v4:
- Added API usage clarifying comments (Caio).

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>