Add multiboot header and boot.asm

src/arch/x86_64/boot.asm

@@ -0,0 +1,8 @@
+global start
+
+section .text
+bits 32
+start:
+    ; print `OK` to screen
+    mov dword [0xb8000], 0x2f4b2f4f
+    hlt

src/arch/x86_64/multiboot_header.asm

@@ -0,0 +1,15 @@
+section .multiboot_header
+header_start:
+    dd 0xe85250d6                ; magic number (multiboot 2)
+    dd 0                         ; architecture 0 (protected mode i386)
+    dd header_end - header_start ; header length
+    ; checksum
+    dd 0x100000000 - (0xe85250d6 + 0 + (header_end - header_start))
+
+    ; insert optional multiboot tags here
+
+    ; required end tag
+    dw 0    ; type
+    dw 0    ; flags
+    dd 8    ; size
+header_end: