x86: Save the BIST value on reset

The built in self test value is available in register eax on start-up. Save
it so that it can be accessed later. Unfortunately we must wait until the
global_data is available before we can do this, so there is a little bit of
shuffling to keep it around.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>