This is support for ctypes on macOS/arm64 based
on PR 21249 by Lawrence D'Anna (Apple).

Changes:
- changed __builtin_available tests from 11.0 to 10.15
- added test to setup.py for ffi_closure_alloc and use
  that in malloc_closure.c
- Minor change in the code path for ffi_prep_closure_var
  (coding style change)

The preprocessor guard in the old version doesn't work, and
isn't really needed (10.4 only supported 32-bit code where
unsigned long is the same as uint32_t).

This also adds an option to stop building after compiling the
3th-party dependencies, as well as a script for archiving those
dependencies. This makes it easier to work on the build.

There are three changes to build-installer.py related to universal2
support:

1. Add 'universal2' information to build-installer.py;
2. Building OpenSSL for arm64 requires a patch at this time;
3. For some reason I had to patch the Tcl build to avoid a build error.

Needed because my previous workaround doesn't work anymore.

This uses a private API, that should be made public later...