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.

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

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
(cherry picked from commit 5afaf99edb)

sysdeps/ieee754/dbl-64/math_config.h

@@ -195,10 +195,11 @@ check_uflow (double x)
 extern const struct exp_data
 {
   double invln2N;
-  double shift;
   double negln2hiN;
   double negln2loN;
   double poly[4]; /* Last four coefficients.  */
+  double shift;
+
   double exp2_shift;
   double exp2_poly[EXP2_POLY_ORDER];
   uint64_t tab[2*(1 << EXP_TABLE_BITS)];