Make function fixed_point_scaling_factor a method of struct type

Make function fixed_point_scaling_factor a method of struct type

This logically connects this function to the object it inspects.

gdb/ChangeLog:

        * gdbtypes.h (struct type) <fixed_point_scaling_factor>: New method,
        replacing fixed_point_scaling_factor.  All callers updated
        throughout this project.
        (fixed_point_scaling_factor): Delete declaration.
        * gdbtypes.c (type::fixed_point_scaling_factor): Replaces
        fixed_point_scaling_factor.  Adjust implementation accordingly.