Add support for the CTF debug format to libabigail.

Add support for the CTF debug format to libabigail.

CTF (C Type Format) is a lightweight debugging format that provides
information about C types and the association between functions and
data symbols and types.  It is designed to be very compact and
simple.  More can be learned about it at https://ctfstd.org.

This patch introduces support in libabigail to extract ABI information
from CTF stored in ELF files.

A few notes on this implementation:

- The implementation is complete in terms of CTF support.  Every CTF
  feature is processed and handled to generate libabigail IR.  This
  includes basic types, typedefs, pointer, array and struct types.
  The CTF record of data objects (variables) and functions are also
  used in order to generate the corresponding libabigail IR artifacts.

- The decoding of CTF data is done using the libctf library which is
  part of binutils.  In order to link with it, binutils shall be built
  with --enable-shared for libctf.so to become available.

- This initial implementation is aimed to simplicity.  We have not
  tried to resolve any and every corner case that may require special
  handling.  We have observed that the DWARF front-end (which is
  naturally way more complex as the scope is way bigger) is plagued
  with hacks to handle such situations.  However, for the CTF support
  we prefer to proceed in a simpler and more modest way: we will
  handle these problems if/when we find them.  The fact that CTF only
  supports C (currently) certainly helps there.

- Likewise, in this basic support we are not handling symbol
  suppressions or other goodies that libabigail provides.  We are new
  to libabigail and ABI analysis, and at this point we simply don't
  have a clear picture about what is most useful/relevant to support
  or not.  With the maintainer's blesssing, we will tackle that
  functionaly after this basic support is applied upstream.

- The implementation in abg-ctf-reader.{cc,h} is pretty much
  self-contained.  As a result there is some duplication in terms of
  ELF handling with the DWARF reader, but since that logic is very
  simple and can be easily implemented, we don't consider this to be a
  big deal (for now.)  Hopefully the maintainers agree.

- The libabigail tools assume that ELF means to always use DWARF to
  generate the ABI IR.  We added a new command-line option --ctf to
  the tools in order to make them to use the CTF debug info instead.
  We are definitely not sure whether this is the best user interface.
  In fact I would be suprised if it was ;)

- We added support for --ctf to both abilint and abidiff.   We are not
  sure whether it would make sense to add support for CTF to the other
  tools.  Feedback welcome.

- We are pondering about what to do in terms of testing.  We have
  cursory tested this implementation using abilint and abidiff.  We
  know we are generating IR corpus that seem to be ok.  It would be
  good however to be able to run the libabigail testsuites using CTF.
  However the testsuites may need some non-trivial changes in order to
  make this possible.  Let's talk about that :)

	* configure.ac: Check for libctf.
	* src/abg-ctf-reader.cc: New file.
	* include/abg-ctf-reader.h: Likewise.
	* src/Makefile.am (libabigail_la_SOURCES): Add abg-ctf-reader.cc
	conditionally.
	* include/Makefile.am (pkginclude_HEADERS): Add abg-ctf-reader.h
	conditionally.
	* tools/abilint.cc (struct options): New option `use_ctf'.
	(display_usage): Documentation for --ctf.
	(parse_command_line): Handle --ctf.
	(main): Honour --ctf.
	* tools/abidiff.cc (struct options): New option `use_ctf'.
	(display_usage): Documentation for --ctf.
	(parse_command_line): Handle --ctf.
	(main): Honour --ctf.
	* doc/manuals/abidiff.rst: Document --ctf.
	* doc/manuals/abilint.rst: Likewise.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>