math: Fix compare sort function on compoundn

math: Fix compare sort function on compoundn

To use the fabs function to the used type, instead of the double
variant.  it fixes a build issue with clang:

./s_compoundn_template.c:64:14: error: absolute value function 'fabs' given an argument of type 'const long double' but has parameter of type 'double' which may cause truncation of value [-Werror,-Wabsolute-value]
   64 |   FLOAT pd = fabs (*(const FLOAT *) p);
      |              ^
./s_compoundn_template.c:64:14: note: use function 'fabsl' instead
   64 |   FLOAT pd = fabs (*(const FLOAT *) p);
      |              ^~~~
      |              fabsl

Reviewed-by: Collin Funk <collin.funk1@gmail.com>