ABI with support for compat 32/64 bits

ABI with support for compat 32/64 bits

The current ABI does not work for compat 32/64 bits. This patch moves
the current ABI as old-abi and provides a new ABI in which all the
structures exchanged between user and kernel-space are packed. Also this
new ABI moves the "int overwrite" member of the struct
lttng_kernel_channel to remove the alignment added by the compiler.

A patch for lttng-modules has been developed in parallel to this one to
support the new ABI. These 2 patches have been tested in all possible
configurations (applied or not) on 64-bit and 32-bit kernels (with
CONFIG_COMPAT) and a user-space in 32 and 64-bit.

Here are the results of the tests :
k 64 compat     | u 32 compat     | OK
k 64 compat     | u 64 compat     | OK
k 64 compat     | u 32 non-compat | KO
k 64 compat     | u 64 non-compat | OK

k 64 non-compat | u 64 compat     | OK
k 64 non-compat | u 32 compat     | KO
k 64 non-compat | u 64 non-compat | OK
k 64 non-compat | u 32 non-compat | KO

k 32 compat     | u compat        | OK
k 32 compat     | u non-compat    | OK

k 32 non-compat | u compat        | OK
k 32 non-compat | u non-compat    | OK

The results are as expected :
- on 32-bit user-space and kernel, every configuration works.
- on 64-bit user-space and kernel, every configuration works.
- with 32-bit user-space on a 64-bit kernel the only configuration
  where it works is when the compat patch is applied everywhere.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>