x86: optimize GDT space

The CPU manual indicates that 8-byte alignment is sufficient,
not sure why gdt_rom was aligned on a 16-byte boundary.

The null descriptor in the GDT is never looked at by the CPU,
save a few bytes by putting the 6-byte pseudo descriptor there.

Change-Id: I73f26cdeb30a91f8258c88ef960a45812a11d959
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>