Kernel/aarch64: Add stubs for Mutex::lock and Mutex::unlock

Also add the g_scheduler_lock to the Dummy.cpp. This makes the aarch64
build succeeded again.

Kernel/Arch/aarch64/Dummy.cpp

@@ -21,6 +21,7 @@
 namespace Kernel {
 
 READONLY_AFTER_INIT Thread* g_finalizer;
+RecursiveSpinlock g_scheduler_lock { LockRank::None };
 
 }
 
@@ -34,6 +35,21 @@ void get_fast_random_bytes(Bytes)
 
 }
 
+// Mutex
+namespace Kernel {
+
+void Mutex::lock(Mode, [[maybe_unused]] LockLocation const& location)
+{
+    VERIFY_NOT_REACHED();
+}
+
+void Mutex::unlock()
+{
+    VERIFY_NOT_REACHED();
+}
+
+}
+
 // Inode
 namespace Kernel {