lttng: Move the UST Callstack analysis to use ILttngUstEventLayout

lttng: Move the UST Callstack analysis to use ILttngUstEventLayout

The UST callstack implementation was defining the event and field
names it's looking for on its own. This is bad and prone to future
breakage.

Instead, integrate these event definitions in the LttngUstEventLayout,
and move the analysis to use these instead.

Change-Id: I098d8569ab2c9c0d2ffd0975aba528a177a02af2
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/50686
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>