Add serialized ABI definition files

This commit contains the serialized ABI definitions for a typical build
of the lttng-ust librairies. This information is extracted using
libabigail (https://sourceware.org/libabigail/).

The artefacts used to generate these were built with CFLAGS="-O0 -ggdb"
and all optional configure switches enabled.

You can compare the serialized ABI with a shared object to check for
changes. For example, here we compare an in-tree built version of
liblttng-ust.so with the serialized ABI of stable-2.13 :

  abidiff \
    extras/abi/2.13/x86_64-pc-linux-gnu/liblttng-ust.so.1.xml \
    src/lib/lttng-ust/.libs/liblttng-ust.so

Change-Id: Ie3b6fa78b156556bb9967bebab36c8e4f563bbe3
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>