linuxboot: fix gdt address calculation

The gdt address calculation in linuxboot.bin is broken in two ways: first
it loads %cs into %eax, but that instruction leaves the high bits of %eax
undefined and we did not clear them.  Secondly, we completely ignore the
incorrect %eax, and use the undefined %ebx instead.

With these issues fixed, linuxboot works again.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit d0652aa8)

pc-bios/optionrom/linuxboot.S

@@ -86,9 +86,10 @@ copy_kernel:
 	/* Now create the GDT descriptor */
 	movw		$((3 * 8) - 1), -16(%bp)
 	mov		%cs, %eax
+	movzwl		%ax, %eax
 	shl		$4, %eax
-	addl		$gdt, %ebx
-	movl		%ebx, -14(%bp)
+	addl		$gdt, %eax
+	movl		%eax, -14(%bp)
 
 	/* And load the GDT */
 	data32 lgdt	-16(%bp)