ptxed: allow printing instructions in ATT format

Add an option (--att) to print the instructions in Linux style ATT
format.

Change-Id: If6e5fbd9b6e1ca6388d78d1ed0f8632dace42aea
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>