ctf: Support changing endianness of a trace, fix bug 453673

ctf: Support changing endianness of a trace, fix bug 453673

The issue is that IOStructGen assumed the endianness of a trace did
not change half way through the tsdl file. Now the elements are progressively
loaded and thus the endinanness is updated as need be.

Change-Id: I8cefbf2f1a841a1decb0722b50cd9ec447bcc2be
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/37424
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Hudson CI