Kernel: Add a beep() syscall that beeps the PC speaker.

Hook this up in Terminal so that the '\a' character generates a beep.
Finally emit an '\a' character in the shell line editing code when
backspacing at the start of the line.

Kernel/Devices/PCSpeaker.h

@@ -0,0 +1,7 @@
+#pragma once
+
+class PCSpeaker {
+public:
+    static void tone_on(int frequency);
+    static void tone_off();
+};