Kernel: Zero out .bss contents on aarch64

After building and running

     objcopy -O binary Build/aarch64/Kernel/Prekernel/Prekernel \
                       /media/sdcard/kernel8.img

things start booting on an actual RPi4 :^)

(Assuming the sdcard contains RPi firmware, an empty config.txt,
and no other kernel*.img files).

Kernel/Prekernel/Arch/aarch64/boot.S

@@ -21,6 +21,14 @@ start:
   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
 
 .globl wait_cycles

Kernel/Prekernel/Arch/aarch64/linker.ld

@@ -29,6 +29,10 @@ SECTIONS
 
     .bss ALIGN(4K) (NOLOAD) : AT (ADDR(.bss))
     {
+	start_of_bss = .;
         *(.bss)
+	end_of_bss = .;
     } :bss
 }
+
+size_of_bss_divided_by_8 = (end_of_bss - start_of_bss) / 8;