libstdc++: Fix std::random_device for avr

This fixes a build failure that affects avr, but could affect other
targets in theory. The _M_fini function should not try to use ::open or
::fopen if _GLIBCXX_USE_DEV_RANDOM is not defined, because no file can
ever have been opened.

libstdc++-v3/ChangeLog:

	* src/c++11/random.cc (random_device::_M_fini): Do not try to
	close the file handle if the target doesn't support the
	/dev/random and /dev/urandom files.