Python 3 compat.

Try to keep our Python sources compatible with both Python 2.7 and 3.

Check with 'pylint --py3k' and 'python -3'.

meta/isa/riscv/defs.py

@@ -3,7 +3,7 @@ RISC-V definitions.
 
 Commonly used definitions.
 """
-
+from __future__ import absolute_import
 from cretonne import TargetISA, CPUMode
 import cretonne.base