[gn build] Change python run lines to python3

macOS 12.3 no longer ships non-3 python.

Almost all of these scripts were launched by ninja, and the GN files
already told it to run them under python3, so this is a fairly small
change.  The main effect is that if you run them manually, you now
get the same behavior.

(A small set of scripts, gn.py, gen.py, sync_source_lists_from_cmake.py,
are for manual running.  For these, it is an actual change.)

Differential Revision: https://reviews.llvm.org/D122345