[patch, libstdc++] Optimize selection sampling by using generator to get two values at once

[Eelis van der Weegen]

This is the same optimization as was recently applied to std::shuffle.

It reduces the runtime of the following program by 20% on my machine:

	int main()
	{
		std::vector<uint64_t> a(10000), b(1000);
		std::mt19937 gen;

		uint64_t c = 0;

		for (int i = 0; i != 1000; ++i)
		{
			std::sample(a.begin(), a.end(), b.begin(), b.size(), gen);
			c += uint64_t(b[32]);
		}

		std::cout << c;
	}
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