Build issues (at least) on arm64 macOS

During the meeting in Limerick, @tla, @eta and I ran into a build issue with C tor on iOS. This was due to an XCode update that arrived while we were in the meeting.

It turns out, this also impacts desktop builds on (at least) arm64 builds.

@mikeperry tested the same method we used to get iOS to build again and had to do a bit more work here:

#!/bin/sh

# libtor.a on MacOS Silicon is thin (arm64 only)
mkdir arm64
mv libtor.a arm64
cd arm64
ar x libtor.a
rm __.SYMDEF\ SORTED
cd ..
libtool -static arm64/*.o -o libtor.a

# libtor-testing.a on MacOS Silicon is thin (arm64 only)
cd src/test
mkdir arm64
mv libtor-testing.a arm64/
cd arm64
ar x libtor-testing.a
rm __.SYMDEF\ SORTED
cd ..
libtool -static arm64/*.o -o libtor-testing.a

We need to make sure to get this into Tor's build system for macOS users. We also need to make sure we don't break setups on x86-64 as well as people on older XCode's? The latter may be hard to do.