x86: Move GDT to a safe location in RAM

Currently, the GDT is either located in FLASH or in the non-relocated
U-Boot image in RAM. Both of these locations are unsafe as those
locations can be erased during a U-Boot update. Move the GDT into the
highest available memory location and relocate U-Boot to just below it

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>