Provide in-kernel headers to make extending kernel easier

Introduce in-kernel headers which are made available as an archive
through proc (/proc/kheaders.tar.xz file). This archive makes it
possible to run eBPF and other tracing programs that need to extend the
kernel for tracing purposes without any dependency on the file system
having headers.

A github PR is sent for the corresponding BCC patch at:
https://github.com/iovisor/bcc/pull/2312

On Android and embedded systems, it is common to switch kernels but not
have kernel headers available on the file system. Further once a
different kernel is booted, any headers stored on the file system will
no longer be useful. This is an issue even well known to distros.
By storing the headers as a compressed archive within the kernel, we can
avoid these issues that have been a hindrance for a long time.

The best way to use this feature is by building it in. Several users
have a need for this, when they switch debug kernels, they do not want to
update the filesyst...