Kernel: Abstract platform-specific serial port access from kprintf

kprintf should not really care about the hardware-specific details of
each UART or serial port out there, so instead of using x86 specific
instructions, let's ensure that we will compile only the relevant code
for debug output for a targeted-specific platform.

Kernel/Arch/aarch64/RPi/DebugOutput.cpp

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2022, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Arch/DebugOutput.h>
+#include <Kernel/Arch/aarch64/RPi/UART.h>
+
+namespace Kernel {
+
+void debug_output(char ch)
+{
+    RPi::UART::the().send(ch);
+}
+
+}