Enable gdb to open Linux kernel dumps

[Ales Novak <alnovak@suse.cz>]

Following patches are adding basic ability to access Linux kernel
dumps using the libkdumpfile library. They're creating new target
"kdump", so all one has to do is to provide appropriate debuginfo and
then run "target kdump /path/to/vmcore".

The tasks of the dumped kernel are mapped to threads in gdb. 

Except for that, there's a code adding understanding of Linux SLAB
memory allocator, which means we can tell for the given address to
which SLAB does it belong, or list objects for give SLAB name - and
more.

Patches are against "gdb-7.10-release" (but will apply elsewhere). 

Note: registers of task are fetched accordingly - either from the dump
metadata (the active tasks) or from their stacks. It should be noted
that as this mechanism varies amongst the kernel versions and
configurations, my naive implementation currently covers only the
dumps I encounter, handling of different kernel versions is to be
added.

In the near future, our plan is to remove the clumsy C-code handling
this and reimplement it in Python - only the binding to certain gdb
structures (e.g. thread, regcache) has to be added. A colleague of
mine is already working on that.

The github home of these patches is at:

https://github.com/alesax/gdb-kdump/tree/for-next

libkdumpfile lives at:

https://github.com/ptesarik/libkdumpfile

Fork adding the SLAB support lives at:

https://github.com/tehcaster/gdb-kdump/tree/slab-support