arch: change panic for Vector traceData to warn_once

Make the traceData panic for Vectors a warn_once.  It's a pity it's
not implemented but it's not a reason to abort the simulation entirely.

Change-Id: I5e97258fd4e3fa385cfe0c4b400524d1f7b154df
Reviewed-on: https://gem5-review.googlesource.com/4860
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/arch/isa_parser.py

@@ -802,7 +802,7 @@ class VecRegOperand(Operand):
 
         wb = '''
         if (traceData) {
-            panic("Vectors not supported yet in tracedata");
+            warn_once("Vectors not supported yet in tracedata");
             /*traceData->setData(final_val);*/
         }
         '''