Fix address of IDT in real-mode entry

Fix address of IDT in real-mode entry

In a case of CS & 0x0fff != 0x0, lidt memory operand does not point
to nullidt, this can raise an exception and shutdown the CPU.

When an AP CPU receives 8-bit Start-Up IPI vector yzH, it starts
execute at physical address 000yz000H. Seems this translates to
either yz00:0000 or y000:z000 (CS:IP), depending of the CPU model.
With the change entry16.inc is relocatable as the commentary suggests
and can be used as ap_sipi_vector on SMP systems.

Change-Id: I885a2888179700ba6e2b11d4f2d6a64ddea4c2dc
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/707
Tested-by: build bot (Jenkins)
Reviewed-by: Idwer Vollering <vidwer@gmail.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>