[ios] Switch to python offset tool (mono/mono#15744)

[ios] Switch to python offset tool (mono/mono#15744)

The C# version doesn't work on 64bit. It would require to update its dependencies, but Zoltan already added a Python version for WebAssembly which works fine.

Some notes:
* changed the semantics of `--xcode-path` in order to match the output of `xcode-select -p`
* switched to python3/pip3
* removed hack for arm64_32, it works properly now because the Python version uses stock `clang`
* removed some leftover usages of `XCODE32_DIR`.

Manually verified by comparing output with the C# version.

This will unblock the iOS team when building on Catalina/Xcode11, as they can't consume SDK archives there. See https://github.com/xamarin/xamarin-macios/pull/6603#issuecomment-512803082

Contributes to https://github.com/mono/mono/issues/9621

Commit migrated from https://github.com/mono/mono/commit/3f7d33ce855dc220873b2e679698f65084dbc336