Fix parameter type in C++ version of iseqsig (bug 23171)

The commit

  commit c85e54ac
  Author: Gabriel F. T. Gomes <gabriel@inconstante.eti.br>
  Date:   Fri Nov 3 10:44:36 2017 -0200

      Provide a C++ version of iseqsig (bug 22377)

mistakenly used double parameters in the long double version of iseqsig,
thus causing spurious conversions to double, as reported on bug 23171.

Tested for powerpc64le and x86_64.