libstdc++: Reduce <random> test iterations for simulators

Some of these tests take several minutes on a simulator like cris-elf,
so we can conditionally run fewer iterations. The testDiscreteDist
helper already supports custom sizes so we just need to make use of that
when { target simulator } matches.

The relevant code is sufficiently tested on other targets, so we're not
losing anything by only running a small number of iterators for sims.

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/random/bernoulli_distribution/operators/values.cc:
	Run fewer iterations for simulator targets.
	* testsuite/26_numerics/random/binomial_distribution/operators/values.cc:
	Likewise.
	* testsuite/26_numerics/random/discrete_distribution/operators/values.cc:
	Likewise.
	* testsuite/26_numerics/random/geometric_distribution/operators/values.cc:
	Likewise.
	* testsuite/26_numerics/random/negative_binomial_distribution/operators/values.cc:
	Likewise.
	* testsuite/26_numerics/random/poisson_distribution/operators/values.cc:
	Likewise.
	* testsuite/26_numerics/random/uniform_int_distribution/operators/values.cc:
	Likewise.