LibCore: Add wrapper for fstatat()

`Core::System::fstatat()` is similar to our standard `Core::System`
wrappers.

`Core::Directory::stat(relative_path, flags)` is a convenience method if
you already have a Directory, to stat a file relative to it.

Userland/Libraries/LibCore/Directory.cpp

@@ -88,6 +88,11 @@ ErrorOr<NonnullOwnPtr<File>> Directory::open(StringView filename, File::OpenMode
     return File::adopt_fd(fd, mode);
 }
 
+ErrorOr<struct stat> Directory::stat(StringView filename, int flags) const
+{
+    return System::fstatat(m_directory_fd, filename, flags);
+}
+
 ErrorOr<struct stat> Directory::stat() const
 {
     return System::fstat(m_directory_fd);