core/x86_64: Enable the NX bit before using it

Spent an embarrassing amount of time trying to figure out why there was a page fault with "reserved bits set". Turns out, I had set the NX bit without enabling it.

core/src/arch/x86_64/platform.zig

@@ -5,10 +5,21 @@ const interrupts = @import("interrupts.zig");
 
 pub const PAGE_SIZE = 4096;
 
+// FIXME: Check if it's supported first.
+fn enableNX() void {
+    asm volatile (
+        \\ mov $0xC0000080, %rcx
+        \\ rdmsr
+        \\ or $0x800, %eax
+        \\ wrmsr
+    );
+}
+
 // Initialize platform-specific components.
 pub fn platformInit() void {
     gdt.setupGDT();
     idt.setupIDT();
+    enableNX();
 }
 
 // Initialize platform-specific components just before beginning multitasking.