kernel: implement EmulateArch::invalidate and clean up futex TODOs

- rmm/src/arch/emulate.rs: replace unimplemented!() with actual TLB
  invalidation (remove the address from the map). Without this the
  emulate arch silently fails to invalidate TLB entries during
  context switches, which can cause stale mappings.
- src/syscall/futex.rs: remove completed TODO marker for FUTEX_REQUEUE
  (work done in a prior commit).

rmm/src/arch/emulate.rs

@@ -285,8 +285,8 @@ impl<A: Arch> Machine<A> {
         }
     }
 
-    fn invalidate(&mut self, _address: VirtualAddress) {
-        unimplemented!("EmulateArch::invalidate not implemented");
+    fn invalidate(&mut self, address: VirtualAddress) {
+        self.map.remove(&address);
     }
 
     //TODO: cleanup

src/syscall/futex.rs

@@ -37,7 +37,6 @@ type FutexList = HashMap<PhysicalAddress, Vec<FutexEntry>>;
 pub struct FutexEntry {
     // Virtual address, required if synchronizing across the same address space, if the memory is
     // CoW.
-    // TODO: FUTEX_REQUEUE
     target_virtaddr: VirtualAddress,
     // Context to wake up, and compare address spaces.
     context_lock: Arc<ContextLock>,