Kernel: Abstract platform-specific current time methods from Scheduler

This change ensures that the scheduler doesn't depend on a platform
specific or arch-specific code when it initializes itself, but rather we
ensure that in compile-time we will generate the appropriate code to
find the correct arch-specific current time methods.

Kernel/Arch/x86/CurrentTime.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2022, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Arch/CurrentTime.h>
+#include <Kernel/Arch/x86/ASM_wrapper.h>
+#include <Kernel/Arch/x86/Processor.h>
+
+namespace Kernel {
+
+static u64 current_time_tsc()
+{
+    return read_tsc();
+}
+
+fptr optional_current_time()
+{
+    VERIFY(Processor::is_initialized()); // sanity check
+    // Figure out a good scheduling time source
+    if (Processor::current().has_feature(CPUFeature::TSC) && Processor::current().has_feature(CPUFeature::CONSTANT_TSC)) {
+        return current_time_tsc;
+    }
+    return nullptr;
+}
+
+}