Fix GDT error offsetting memory by some bytes

The error in the GDT caused the whole memory space to be offset by
128B or 256B (It's not entirely sure which one of those). This was
only made visible by later changes, but now it's fixed.

i386/loader/src-asm/memory/gdt.asm

@@ -67,7 +67,7 @@ make_regular_data_segment_in_gdt:
 
 .write_base_address:
     mov [eax + 2], bx
-    shr bx, 16
+    shr ebx, 16
     mov [eax + 4], bl
     mov [eax + 7], bh
 
@@ -282,7 +282,7 @@ make_code_segment_in_gdt:
 
 .write_base_address:
     mov [eax + 2], bx
-    shr bx, 16
+    shr ebx, 16
     mov [eax + 4], bl
     mov [eax + 7], bh
 
@@ -389,6 +389,9 @@ load_flat_gdt:
     or ebx, 1
     mov cr0, ebx
 
+    xor bx, bx
+    mov ds, bx
+
     jmp dword 0x08:.epilog
 
 bits 32
@@ -407,7 +410,7 @@ bits 32
 align 16
 .gdtr:
 .gdtr.length:
-    dw 23
+    dw 15
 .gdtr.pointer:
     dd GDT_MEMORY_AREA