arch/x86: Add interrupt and segment descriptor tables

This adds a rough-and-ready implementation for the segmentation and
interrupt descriptor tables.
arch/x86: Add a PIT interface and implementation
arch/x86: Add a backtrace mechaniscm

This walks up the stack, printing the location of each stack frame.
kernel: Print a backtrace on kernel panic
tools: Add a headercheck script

This script generates a list of all header files, and then iterates over
each header and source file, listing those which include the header. The
idea here is that it provides a convenient means for checking to see how
widely used a source file is. For example, this tool could be used as a
reference before changing a function prototype, to see how many source
files will be affected.