tools: update shebang in thumb-cc.py
Some distros, including Ubuntu/Debian, don't provide a "python"
command unless an additional package is installed. Using "python3"
explicitly should work everywhere.
Using "/usr/bin/env" respects the user's PATH, so they can choose
a specific Python version instead of forcing the systemwide copy.
Change-Id: I02e3ead92939c42df6def79dec3dc2a62c62b1c3