Kernel: Introduce basic pre-kernel environment

This implements a simple bootloader that is capable of loading ELF64
kernel images. It does this by using QEMU/GRUB to load the kernel image
from disk and pass it to our bootloader as a Multiboot module.

The bootloader then parses the ELF image and sets it up appropriately.
The kernel's entry point is a C++ function with architecture-native
code.

Co-authored-by: Liav A <liavalb@gmail.com>

Kernel/Arch/x86/x86_64/Boot/ap_setup.S

@@ -1,6 +1,19 @@
-#define _BOOTLOADER
 #include <Kernel/Sections.h>
 
+.section .text
+
+.global gdt64ptr
+gdt64ptr:
+#if ARCH(X86_64)
+.quad 0
+#else
+.int 0
+#endif
+
+.global code64_sel
+code64_sel:
+.short 0
+
 .extern init_ap
 .type init_ap, @function
 
@@ -92,16 +105,15 @@ apic_ap_start32:
     movl %eax, %cr0
 
     /* load the temporary 64-bit gdt from boot that points above 3GB */
-    mov $(gdt64ptr - KERNEL_BASE), %eax
+    // FIXME: uncomment this
+    //mov gdt64ptr, %eax
     lgdt (%eax)
 
     /* jump above 3GB into our identity mapped area now */
-    ljmpl $code64_sel, $(apic_ap_start64 - apic_ap_start + 0xc0008000)
+    // FIXME: this assumes that code64_sel is always 8
+    ljmpl $8, $(apic_ap_start64 - apic_ap_start + 0xc0008000)
 .code64
 apic_ap_start64:
-    movabs $gdt64ptr, %rax
-    lgdt (%rax)
-
     mov $0, %ax
     mov %ax, %ss
     mov %ax, %ds
@@ -129,7 +141,8 @@ apic_ap_start64:
 
     /* push the Processor pointer this CPU is going to use */
     movq (ap_cpu_init_processor_info_array - apic_ap_start)(%ebp), %rax
-    movq $KERNEL_BASE, %r8
+    movabsq $(kernel_base), %r8
+    movq (%r8), %r8
     addq %r8, %rax
     movq 0(%rax, %rsi, 4), %rax
     push %rax