x86: introduce new segmentation.h header

This header has a bunch of data structure definitions and macros useful
for manipulating segment descriptors on X86. The old IDT_ENTRY defintion
is removed in favor of the new 'struct segment_descriptor' which can be
used for all segment descriptor types and not just IRQ gates.

We also add some inline helper functions for examining segment registers,
descriptor tables, and doing far jumps/calls.

Change-Id: I640879073afa9765d2a214c3fb3c3305fef94b5e
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>