lib: Add halffloat implementation

Probe for and make an API available for tests to use f16c intrinsics to
generate needed fp16 pixel data.

Also import a pure c fp32 <-> fp16 conversion implementation from Mesa
18.3.4, which will act as a fallback when f16c is unavailable.

rfc2:
- Change API to reduce number of function calls (Maarten)

v1:
- Move pragma so AVX code isn't emitted for fallbacks (Ville)
- Change edx to ecx (Ville)

Signed-off-by: Kevin Strasser <kevin.strasser@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>