[libc++] Fix filesystem::remove_all() on FreeBSD (#79540)

remove_all_impl() opens the target path with O_NOFOLLOW, which fails if
the target is a symbolic link. On FreeBSD, rather than returning ELOOP,
openat() returns EMLINK. This is unlikely to change for compatibility
reasons, see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214633 .

Thus, check for EMLINK as well.