x86: Tidy up global_data flags

These flags now overlap some global ones. Adjust the x86-specific flags to
avoid this. Since this requires a change to the start.S code, add a way for
tools to find the 32-bit cold reset entry point. Previously this was at a
fixed offset.

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