Move kernel to higher half.

Return to having the bootloader re-map the kernel into the higher
half before jumping into the kernel entrypoint, so we don't have
to juggle pointers inside the kernel.

src/arch/x86_64/kernel.ld

@@ -1,7 +1,8 @@
 ENTRY(_start)
 SECTIONS
 {
-	. = 0x100000;
+	OFFSET = 0xFFFF800000000000;
+	. = OFFSET + 0x100000;
 
 	.header : {
 		header = .;