Kernel: Move ScopedCritical.cpp to Kernel base directory

This file does not contain any architecture specific implementations,
so we can move it to the Kernel base directory. Also update the relevant
include paths.

Kernel/ScopedCritical.cpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/ScopedCritical.h>
+
+#include <Kernel/Arch/Processor.h>
+
+namespace Kernel {
+
+ScopedCritical::ScopedCritical()
+{
+    enter();
+}
+
+ScopedCritical::~ScopedCritical()
+{
+    if (m_valid)
+        leave();
+}
+
+ScopedCritical::ScopedCritical(ScopedCritical&& from)
+    : m_valid(exchange(from.m_valid, false))
+{
+}
+
+ScopedCritical& ScopedCritical::operator=(ScopedCritical&& from)
+{
+    if (&from != this) {
+        m_valid = exchange(from.m_valid, false);
+    }
+    return *this;
+}
+
+void ScopedCritical::leave()
+{
+    VERIFY(m_valid);
+    m_valid = false;
+    Processor::leave_critical();
+}
+
+void ScopedCritical::enter()
+{
+    VERIFY(!m_valid);
+    m_valid = true;
+    Processor::enter_critical();
+}
+
+}