libstdc++: Fix test that fails on Solaris [PR104731]

libstdc++: Fix test that fails on Solaris [PR104731]

On Solaris the dirent::d_name member is a single char, causing this test
to fail with warnings about buffer overflow. Change the test to use a
union with additional space for writing a string to the d_name member.

libstdc++-v3/ChangeLog:

	PR libstdc++/104731
	* testsuite/27_io/filesystem/iterators/error_reporting.cc:
	Use a trailing char array as storage for dirent::d_name.

(cherry picked from commit aa8bdfee1db818b9a56908ab0197ff02c54bf281)