Kernel: Make sure we free the thread stack on thread exit

This adds two new arguments to the thread_exit system call which let
a thread unmap an arbitrary VM range on thread exit. LibPthread
uses this functionality to unmap the thread stack.

Fixes #7267.

Kernel/Syscall.cpp

@@ -97,7 +97,7 @@ KResultOr<FlatPtr> handle(RegisterState& regs, FlatPtr function, FlatPtr arg1, F
             process.sys$exit(arg1);
             break;
         case SC_exit_thread:
-            process.sys$exit_thread(arg1);
+            process.sys$exit_thread(arg1, arg2, arg3);
             break;
         default:
             VERIFY_NOT_REACHED();