move gdt, use fixed addr

stage_3/src/main.rs

@@ -35,7 +35,7 @@ pub extern "C" fn _start() -> ! {
         com::try_com1_init();
     }
 
-    let gdt_ptr = ptr::with_exposed_provenance::<u64>(0xc000);
+    let gdt_ptr = ptr::with_exposed_provenance::<u64>(0xe000);
     let gdt_slice = unsafe { slice::from_raw_parts(gdt_ptr, 4) };
 
     vga_println!("hello from rust :)");