[prefix] Use 16-bit protected mode for access to high memory

[prefix] Use 16-bit protected mode for access to high memory

Flat real mode works perfectly on real hardware, but seems to cause
problems for some hypervisors.  Revert to using 16-bit protected mode
(and returning to real mode with 4GB limits, so as not to break PMM
BIOSes).

Allow the code specific to the .mrom format to continue to assume that
flat real mode works, since this format is specific to real hardware.

Signed-off-by: Michael Brown <mcb30@ipxe.org>