Kernel: Reset profiling state on exec() (but keep it going)

We now log the new executable on exec() and throw away all the samples
we've accumulated so far. But profiling keeps going.

Kernel/Profiling.cpp

@@ -90,6 +90,12 @@ void stop()
     s_process = nullptr;
 }
 
+void did_exec(const String& new_executable_path)
+{
+    executable_path() = new_executable_path;
+    s_next_slot_index = 0;
+}
+
 void for_each_sample(Function<void(Sample&)> callback)
 {
     for (size_t i = 0; i < s_next_slot_index; ++i) {