Kernel: Make the kernel compile & link for x86_64

It's now possible to build the whole kernel with an x86_64 toolchain.
There's no bootstrap code so it doesn't work yet (obviously.)

Kernel/Arch/x86_64/Boot/boot.S

@@ -0,0 +1,183 @@
+.set MULTIBOOT_MAGIC,         0x1badb002
+.set MULTIBOOT_PAGE_ALIGN,    0x1
+.set MULTIBOOT_MEMORY_INFO,   0x2
+.set MULTIBOOT_VIDEO_MODE,    0x4
+.set multiboot_flags,         MULTIBOOT_PAGE_ALIGN | MULTIBOOT_MEMORY_INFO | MULTIBOOT_VIDEO_MODE
+.set multiboot_checksum,      -(MULTIBOOT_MAGIC + multiboot_flags)
+
+.section .multiboot
+.align 4
+
+.long MULTIBOOT_MAGIC
+.long multiboot_flags
+.long multiboot_checksum
+
+
+/* for MULTIBOOT_MEMORY_INFO */
+.long 0x00000000    /* header_addr */
+.long 0x00000000    /* load_addr */
+.long 0x00000000    /* load_end_addr */
+.long 0x00000000    /* bss_end_addr */
+.long 0x00000000    /* entry_addr */
+
+/* for MULTIBOOT_VIDEO_MODE */
+.long 0x00000000    /* mode_type */
+.long 1280          /* width */
+.long 1024          /* height */
+.long 32            /* depth */
+
+.section .stack, "aw", @nobits
+stack_bottom:
+.skip 32768
+stack_top:
+
+.global kernel_cmdline
+kernel_cmdline:
+.skip 4096
+
+.section .page_tables, "aw", @nobits
+.align 4096
+.global boot_pdpt
+boot_pdpt:
+.skip 4096
+.global boot_pd0
+boot_pd0:
+.skip 4096
+.global boot_pd3
+boot_pd3:
+.skip 4096
+.global boot_pd0_pt0
+boot_pd0_pt0:
+.skip 4096 * 4
+.global boot_pd3_pts
+boot_pd3_pts:
+.skip 4096 * 8
+.global boot_pd3_pt1023
+boot_pd3_pt1023:
+.skip 4096
+
+.section .text
+
+.global start
+.type start, @function
+
+.extern init
+.type init, @function
+
+.extern multiboot_info_ptr
+.type multiboot_info_ptr, @object
+
+start:
+    cli
+    cld
+
+    /* We don't know where the bootloader might have put the command line.
+     * It might be at an inconvenient location that we're not about to map,
+     * so let's just copy it to a convenient location while we have the whole
+     * memory space identity-mapped anyway. :^)
+     */
+
+    movl %ebx, %esi
+    addl $16, %esi
+    movl (%esi), %esi
+    movl $1024, %ecx
+    movl $(kernel_cmdline - 0xc0000000), %edi
+    rep movsl
+
+    call init
+    add $4, %esp
+
+    cli
+loop:
+    hlt
+    jmp loop
+
+.extern init_ap
+.type init_ap, @function
+
+/*
+  The apic_ap_start function will be loaded to P0x00008000 where the APIC
+  will boot the AP from in real mode. This code also contains space for
+  special variables that *must* remain here. When initializing the APIC,
+  the code here gets copied to P0x00008000, the variables in here get
+  populated and then the the boot of the APs will be triggered. Having
+  the variables here allows us to access them from real mode. Also, the
+  code here avoids the need for relocation entries.
+
+  Basically, the variables between apic_ap_start and end_apic_ap_start
+  *MUST* remain here and cannot be moved into a .bss or any other location.
+*/
+.global apic_ap_start
+.type apic_ap_start, @function
+apic_ap_start:
+.code16
+    cli
+    jmp $0x800, $(1f - apic_ap_start) /* avoid relocation entries */
+1:
+    mov %cs, %ax
+    mov %ax, %ds
+
+    xor %ax, %ax
+    mov %ax, %sp
+
+    /* load the first temporary gdt */
+    lgdt (ap_cpu_gdtr_initial - apic_ap_start)
+
+    /* enable PM */
+    movl %cr0, %eax
+    orl $1, %eax
+    movl %eax, %cr0
+
+    ljmpl $8, $(apic_ap_start32 - apic_ap_start + 0x8000)
+apic_ap_start32:
+.code32
+    cli
+    hlt
+
+.align 4
+.global apic_ap_start_size
+apic_ap_start_size:
+    .2byte end_apic_ap_start - apic_ap_start
+.align 4
+ap_cpu_id:
+    .4byte 0x0
+ap_cpu_gdt:
+    /* null */
+    .8byte 0x0
+    /* code */
+    .4byte 0x0000FFFF
+    .4byte 0x00cf9a00
+    /* data */
+    .4byte 0x0000FFFF
+    .4byte 0x00cf9200
+ap_cpu_gdt_end:
+ap_cpu_gdtr_initial:
+    .2byte ap_cpu_gdt_end - ap_cpu_gdt - 1
+    .4byte (ap_cpu_gdt - apic_ap_start) + 0x8000
+ap_cpu_gdtr_initial2:
+    .2byte ap_cpu_gdt_end - ap_cpu_gdt - 1
+    .4byte (ap_cpu_gdt - apic_ap_start) + 0xc0008000
+.global ap_cpu_gdtr
+ap_cpu_gdtr:
+    .4byte 0x0 /* will be set at runtime */
+.global ap_cpu_idtr
+ap_cpu_idtr:
+    .4byte 0x0 /* will be set at runtime */
+.global ap_cpu_init_cr0
+ap_cpu_init_cr0:
+    .4byte 0x0 /* will be set at runtime */
+.global ap_cpu_init_cr3
+ap_cpu_init_cr3:
+    .4byte 0x0 /* will be set at runtime */
+.global ap_cpu_init_cr4
+ap_cpu_init_cr4:
+    .4byte 0x0 /* will be set at runtime */
+.global ap_cpu_init_processor_info_array
+ap_cpu_init_processor_info_array:
+    .4byte 0x0 /* will be set at runtime */
+.global ap_cpu_init_stacks
+ap_cpu_init_stacks:
+    /* array of allocated stack pointers */
+    /* NOTE: ap_cpu_init_stacks must be the last variable before
+             end_apic_ap_start! */
+.set end_apic_ap_start, .