Everywhere: Move global Kernel pattern code to Kernel/Library directory

This has KString, KBuffer, DoubleBuffer, KBufferBuilder, IOWindow,
UserOrKernelBuffer and ScopedCritical classes being moved to the
Kernel/Library subdirectory.

Also, move the panic and assertions handling code to that directory.

Kernel/Library/Panic.cpp

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Format.h>
+#include <Kernel/Arch/Processor.h>
+#if ARCH(X86_64)
+#    include <Kernel/Arch/x86_64/Shutdown.h>
+#elif ARCH(AARCH64)
+#    include <Kernel/Arch/aarch64/RPi/Watchdog.h>
+#endif
+#include <Kernel/CommandLine.h>
+#include <Kernel/KSyms.h>
+#include <Kernel/Library/Panic.h>
+#include <Kernel/Tasks/Thread.h>
+
+namespace Kernel {
+
+[[noreturn]] static void __shutdown()
+{
+#if ARCH(X86_64)
+    qemu_shutdown();
+    virtualbox_shutdown();
+#elif ARCH(AARCH64)
+    RPi::Watchdog::the().system_shutdown();
+#endif
+    // Note: If we failed to invoke platform shutdown, we need to halt afterwards
+    // to ensure no further execution on any CPU still happens.
+    Processor::halt();
+}
+
+void __panic(char const* file, unsigned int line, char const* function)
+{
+    // Avoid lock ranking checks on crashing paths, just try to get some debugging messages out.
+    auto* thread = Thread::current();
+    if (thread)
+        thread->set_crashing();
+
+    critical_dmesgln("at {}:{} in {}", file, line, function);
+    dump_backtrace(PrintToScreen::Yes);
+    if (!CommandLine::was_initialized())
+        Processor::halt();
+    switch (kernel_command_line().panic_mode()) {
+    case PanicMode::Shutdown:
+        __shutdown();
+    case PanicMode::Halt:
+        [[fallthrough]];
+    default:
+        Processor::halt();
+    }
+}
+}