Kernel/aarch64: Stub out atomic SafeMem functions

These are used in futexes, which are needed if we want to get further in
`run-tests`.

For now, we have no way to return a non-fatal error if an access fault
is raised while executing these, so the kernel will panic. Some would
consider this a DoS vulnerability where a malicious userspace app can
crash the kernel by passing bogus pointers to it, but I prefer to call
it progress :^)

Kernel/Arch/aarch64/SafeMem.cpp

@@ -29,34 +29,35 @@ bool safe_memcpy(void* dest_ptr, void const* src_ptr, unsigned long n, void*&)
     return true;
 }
 
-Optional<bool> safe_atomic_compare_exchange_relaxed(u32 volatile*, u32&, u32)
+Optional<bool> safe_atomic_compare_exchange_relaxed(u32 volatile* var, u32& expected, u32 val)
 {
-    TODO_AARCH64();
-    return {};
+    // FIXME: Handle access faults.
+    return AK::atomic_compare_exchange_strong(var, expected, val, AK::memory_order_relaxed);
 }
 
-Optional<u32> safe_atomic_load_relaxed(u32 volatile*)
+Optional<u32> safe_atomic_load_relaxed(u32 volatile* var)
 {
-    TODO_AARCH64();
-    return {};
+    // FIXME: Handle access faults.
+    return AK::atomic_load(var, AK::memory_order_relaxed);
 }
 
-Optional<u32> safe_atomic_fetch_add_relaxed(u32 volatile*, u32)
+Optional<u32> safe_atomic_fetch_add_relaxed(u32 volatile* var, u32 val)
 {
-    TODO_AARCH64();
-    return {};
+    // FIXME: Handle access faults.
+    return AK::atomic_fetch_add(var, val, AK::memory_order_relaxed);
 }
 
-Optional<u32> safe_atomic_exchange_relaxed(u32 volatile*, u32)
+Optional<u32> safe_atomic_exchange_relaxed(u32 volatile* var, u32 val)
 {
-    TODO_AARCH64();
-    return {};
+    // FIXME: Handle access faults.
+    return AK::atomic_exchange(var, val, AK::memory_order_relaxed);
 }
 
-bool safe_atomic_store_relaxed(u32 volatile*, u32)
+bool safe_atomic_store_relaxed(u32 volatile* var, u32 val)
 {
-    TODO_AARCH64();
-    return {};
+    // FIXME: Handle access faults.
+    AK::atomic_store(var, val);
+    return true;
 }
 
 bool handle_safe_access_fault(RegisterState&, FlatPtr)