math: Improve layout of expf data

math: Improve layout of expf data

GCC aligns global data to 16 bytes if their size is >= 16 bytes.  This patch
changes the exp2f_data struct slightly so that the fields are better aligned.
As a result on targets that support them, load-pair instructions accessing
poly_scaled and invln2_scaled are now 16-byte aligned.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
(cherry picked from commit 44fa9c1080fe6a9539f0d2345b9d2ae37b8ee57a)