x86: Override calculate_relocation_address to use the e820 map

Because calculate_relocation_address now uses the e820 map, it will be able
to avoid addresses over 32 bits and regions that are at high addresses but
not big enough for U-Boot. It also means we can remove the hack which
limitted U-Boot's idea of the size of memory to less than 4GB.

Also take into account the space needed for the heap and stack, so we avoid
picking a very small region those areas might overlap with something it
shouldn't.

Signed-off-by: Gabe Black <gabeblack@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>