x86: don't shift data addresses between builds

Inserting the IDT results in any data afterwards being shifted.
We want the memory addresses between the zephyr_prebuilt.elf
and zephyr.elf to be as close as possible. Insert some dummy
data in the linker script the same size as the gen_idt data
structures. Needed for forthcoming patches which generate MMU
page tables at build time.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>