exp2 suffices here. Some trivial speedup is done in addition here by
reusing results.

This retains accuracy, and in particular results in identical values
with GNU libm + gcc/clang.

sample benchmark (Haswell, GNU/Linux):
proposed : 424160 decicycles in pow_table,     512 runs,      0 skips
exp2 only: 1262093 decicycles in pow_table,     512 runs,      0 skips
old      : 2849085 decicycles in pow_table,     512 runs,      0 skips
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>