math: Improve layout of exp/exp10 data

math: Improve layout of exp/exp10 data

GCC aligns global data to 16 bytes if their size is >= 16 bytes.  This patch
changes the exp_data struct slightly so that the fields are better aligned
and without gaps.  As a result on targets that support them, more load-pair
instructions are used in exp.  Exp10 is improved by moving invlog10_2N later
so that neglog10_2hiN and neglog10_2loN can be loaded using load-pair.

The exp benchmark improves 2.5%, "144bits" by 7.2%, "768bits" by 12.7% on
Neoverse V2.  Exp10 improves by 1.5%.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>