libstdc++: Handle EMLINK and EFTYPE in std::filesystem::remove_all

Although POSIX requires ELOOP, FreeBSD documents that openat with
O_NOFOLLOW returns EMLINK if the last component of a filename is a
symbolic link.  Check for EMLINK as well as ELOOP, so that the TOCTTOU
mitigation in remove_all works correctly.

See https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214633 or the
FreeBSD man page for reference.

According to its man page, DragonFlyBSD also uses EMLINK for this error,
and NetBSD uses its own EFTYPE. OpenBSD follows POSIX and uses EMLINK.

This fixes these failures on FreeBSD:
FAIL: 27_io/filesystem/operations/remove_all.cc  -std=gnu++17 execution test
FAIL: experimental/filesystem/operations/remove_all.cc  -std=gnu++17 execution test

libstdc++-v3/ChangeLog:

	* src/c++17/fs_ops.cc (remove_all) [__FreeBSD__ || __DragonFly__]:
	Check for EMLINK as well as ELOOP.
	[__NetBSD__]: Check for EFTYPE as well as ELOOP.