x86/boot: enable Page Size Extensions

This single fucking bit is the reason 4MB pages
weren't working all along

arch/x86/boot/multiboot.S

@@ -5,6 +5,8 @@
 #include <arch/multiboot.h>
 #include <arch/page.h>
 
+	.code32
+
 	/* see arch/x86/config/kernel.ld */
 	.extern _image_start_phys
 	.extern _kernel_end_phys
@@ -202,6 +204,11 @@ ASM_ENTRY(_start)
 	 */
 	movl	$(phys_addr(pd0) + 0x003), phys_addr(pd0) + 1023 * 4 /* 0xffc00000 */
 
+	/* set the Page Size Extensions bit in cr4 */
+	mov	%cr4, %ecx
+	or	$0x00000010, %ecx
+	mov	%ecx, %cr4
+
 	/* put the (physical) address of pd0 into cr3 so it will be used */
 	mov	$phys_addr(pd0), %ecx
 	mov	%ecx, %cr3