Kernel: Let aarch64 port call into C++

Put all but the first core into a loop, make room for some stack,
and call init().

Kernel/Prekernel/Arch/aarch64/boot.S

@@ -9,7 +9,18 @@
 .global start
 .type start, @function
 start:
-  b Lhalt
+  // Let only core 0 continue, put other cores to sleep.
+  mrs x13, MPIDR_EL1
+  and x13, x13, 0xff
+  cbnz x13, Lhalt
+
+  // Let stack start before .text for now.
+  // 512 kiB (0x8000) of stack are probably not sufficient, especially once we give the other cores some stack too,
+  // but for now it's ok.
+  ldr x14, =start
+  mov sp, x14
+
+  b init
 
 Lhalt:
   wfi

Kernel/Prekernel/Arch/aarch64/init.cpp

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2021, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+extern "C" [[noreturn]] void init();
+extern "C" [[noreturn]] void init()
+{
+    for (;;) { }
+}

Kernel/Prekernel/CMakeLists.txt

@@ -3,6 +3,7 @@ set(SOURCES
 if ("${SERENITY_ARCH}" STREQUAL "aarch64")
     set(SOURCES
         ${SOURCES}
+        Arch/aarch64/init.cpp
         Arch/aarch64/boot.S
     )
 else()