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
llvm-config output with the gn build is just good enough to make
tests pass, but llvm-config wants all .a files it knows about to
actually exist. So let it know about fewer .a files that don't
exist if not all targets are enabled.
