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

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.

Author: markjdb

libcxx/src/filesystem/operations.cpp

@@ -815,8 +815,9 @@ uintmax_t remove_all_impl(int parent_directory, const path& p, error_code& ec) {
 
   // If opening `p` failed because it wasn't a directory, remove it as
   // a normal file instead. Note that `openat()` can return either ENOTDIR
-  // or ELOOP depending on the exact reason of the failure.
-  if (ec == errc::not_a_directory || ec == errc::too_many_symbolic_link_levels) {
+  // or ELOOP depending on the exact reason of the failure. On FreeBSD it
+  // may return EMLINK instead of ELOOP, contradicting POSIX.
+  if (ec == errc::not_a_directory || ec == errc::too_many_symbolic_link_levels || ec == errc::too_many_links) {
     ec.clear();
     if (::unlinkat(parent_directory, p.c_str(), /* flags = */ 0) == -1) {
       ec = detail::capture_errno();