util: add float to float16 conversions with RTZ and RTNE

util: add float to float16 conversions with RTZ and RTNE

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

v2:
- Replace custom f32 -> f16 RTZ implementation with the softfloat
  one (Andres).

v3:
- 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>