[NFC] convert compiler-rt/lib/dfsan/scripts/build-libc-list.py to python3 syntax

I found a few Python files not using Python 3 syntax in the tree
when trying to apply reformatting to them. This was converted with
2to3 and the changes seemed sane.

Reviewed By: MaskRay, browneee

Differential Revision: https://reviews.llvm.org/D143701