Kernel: Move the aarch64 boot.S out of the Prekernel directory

Kernel/Arch/aarch64/boot.S

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+// In a specially-named text section so that the linker script can put it first in .text.
+.section ".text.first"
+
+.global start
+.type start, @function
+start:
+  // Let only core 0 continue, put other cores to sleep.
+  mrs x13, MPIDR_EL1
+  and x13, x13, 0xff
+  cbnz x13, _ZN6Kernel9Processor4haltEv
+
+  // Let stack start before .text for now.
+  // 512 kiB (0x80000) of stack are probably not sufficient, especially once we give the other cores some stack too,
+  // but for now it's ok.
+  msr SPSel, #0   //Use the same SP as we descend into EL1
+  ldr x14, =start
+  mov sp, x14
+
+  // Clear BSS.
+  ldr x14, =start_of_bss
+  ldr x15, =size_of_bss_divided_by_8
+Lbss_clear_loop:
+  str xzr, [x14], #8
+  subs x15, x15, #1
+  bne Lbss_clear_loop
+
+  b init