libstdc++: Make std::random_device throw std::system_error [PR105081]

libstdc++: Make std::random_device throw std::system_error [PR105081]

This changes std::random_device constructors to throw std::system_error
(with EINVAL as the error code) when the constructor argument is
invalid. We can also throw std::system_error when read(2) fails so that
the exception includes the additional information provided by errno.

As noted in the PR, this is consistent with libc++, and doesn't break
any existing code which catches std::runtime_error, because those
handlers will still catch std::system_error.

libstdc++-v3/ChangeLog:

	PR libstdc++/105081
	* src/c++11/random.cc (__throw_syserr): New function.
	(random_device::_M_init, random_device::_M_init_pretr1): Use new
	function for bad tokens.
	(random_device::_M_getval): Use new function for read errors.
	* testsuite/util/testsuite_random.h (random_device_available):
	Change catch handler to use std::system_error.