Univariate GCD timing: use sr_gcd when appropriate.

The benchmark consists of two parts:
1) timing of different GCD calculation methods (i.e. subresultant PRS,
   heuristic, chinese remaindering).
2) timing of different implementations of the same method. The purpose
   is to find out how (in)efficient GiNaC::ex is as a representation
   of (univariate) polynomials (as a side note, the result is a bit
   depressing -- using coefficient vector instead of GiNaC:ex makes
   GCD calculation 50x -- 1000x faster).

Now GiNaC uses modular (chinese remaindering) GCD by default, so part 2)
got broken, i.e. instead of (intended) timings

a) (heuristic, GiNaC::ex) versus (heuristic, coefficient vector)
b) (PRS, GiNaC::ex) versus (PRS, coefficient vector)

one gets

a') (heuristic, GiNaC::ex) versus (heuristic, coefficient vector)
b') (chinese remaindering, GiNaC::ex) versus (PRS, coefficient vector)

Set the gcd_options::use_sr_gcd to restore the meaning of the benchmark.

Note: this patch does not affect the library proper.