GBE: use native exp instruction when enough precision

GBE: use native exp instruction when enough precision

for the input data with enough precision, use the native exp instruction,
otherwise, use the software path to emulate the exp function.

Signed-off-by: Guo Yejun <yejun.guo@intel.com>
Reviewed-by: "Song, Ruiling" <ruiling.song@intel.com>