[kernel] Remove obsolete 1MiB offset

Back when the bootloader could only mirror higher half memory linearly
to the lower half, the kernel couldn't be loaded at the beginning of
kernel space because it was unlikely to find enough pages there, so it
was loaded at +1MiB in kernel space. Now the bootloader can map any
pages to the necessary locations, the offset can be removed.

Tags: elf linker virtual memory

src/arch/x86_64/kernel.ld

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