Kernel: Adjust aarch64 linker script

- .text now starts at 0x80000, where an actual (non-qemu) RPi expects
- use magic section name ".text.first" to make sure the linker script
  puts the kernel entry point at the start of the .text section
- remove a few things from the x86 linker script that aren't needed
  for aarch64 (yet?)

Kernel/Prekernel/Arch/aarch64/boot.S

@@ -4,7 +4,8 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-.text
+// 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

Kernel/Prekernel/Arch/aarch64/linker.ld

@@ -2,7 +2,6 @@ ENTRY(start)
 
 PHDRS
 {
-  boot_text PT_LOAD ;
   text PT_LOAD ;
   data PT_LOAD ;
   bss PT_LOAD ;
@@ -10,18 +9,11 @@ PHDRS
 
 SECTIONS
 {
-    . = 0x00100000;
-
-    start_of_prekernel_image = .;
-
-    .boot_text ALIGN(4K) : AT (ADDR(.boot_text))
-    {
-        KEEP(*(.multiboot))
-    } :boot_text
+    . = 0x00080000;
 
     .text ALIGN(4K) : AT (ADDR(.text))
     {
-        start_of_prekernel_text = .;
+        *(.text.first)
         *(.text*)
     } :text
 
@@ -37,11 +29,6 @@ SECTIONS
 
     .bss ALIGN(4K) (NOLOAD) : AT (ADDR(.bss))
     {
-        *(COMMON)
         *(.bss)
-        *(.stack)
-        *(.page_tables)
     } :bss
-
-    end_of_prekernel_image = .;
 }