libstdc++: Implement std::random_device::entropy() for other sources

libstdc++: Implement std::random_device::entropy() for other sources

Currently this function only returns a non-zero value for /dev/random
and /dev/urandom. When a hardware instruction such as RDRAND is in use
it should (in theory) be perfectly random and produce 32 bits of entropy
in each 32-bit result. Add a helper function to identify the source of
randomness from the _M_func and _M_file data members, and return a
suitable value when RDRAND or RDSEED is being used.

libstdc++-v3/ChangeLog:

	* src/c++11/random.cc (which_source): New helper function.
	(random_device::_M_getentropy()): Use which_source and return
	suitable values for sources other than device files.
	* testsuite/26_numerics/random/random_device/entropy.cc: New test.