libstdc++: Make spurious std::random_device FAIL less likely

libstdc++: Make spurious std::random_device FAIL less likely

It's possible that independent reads from /dev/random and /dev/urandom
could produce the same value by chance. Retry if that happens. The
chances of it happening twice are miniscule.

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/random/random_device/cons/token.cc:
	Retry if random devices produce the same value.