[compiler-rt] Unify python shebangs (#187285)

As per PEP-0394[1], there is no real concensus over what binary names
Python has, specifically 'python' could be Python 3, Python 2, or not
exist.

However, everyone has a python3 interpreter and the scripts are all
written for Python 3. Unify the shebangs so that the ~50% of shebangs
that use python now use python3.

[1] https://peps.python.org/pep-0394/

GitOrigin-RevId: 015e3d2b20924e966345d3c493fb16f2fa3160b4