Another bug Eric noticed. I had tests for create_directories() but not
create_directory(), and the former was already checking whether it
existed or not, so I didn't notice this bug.

	PR libstdc++/71036
	* src/filesystem/ops.cc (create_dir): Handle EEXIST from mkdir.
	* testsuite/experimental/filesystem/operations/create_directory.cc:
	New test.

Tested x86_64-linux, committed to trunk. Backports to follow.