Add toochain definition for Python 3.14.

[yilei]

Why? This is required to build wheels for Python 3.14, part of #58.

The Python 3.14 build seems to just work. The Python 3.14 free-threaded version builds, but not working yet, at least on mac:

❯ python -c 'import sys; print(f"{sys.version=}")'
sys.version='3.14.2 free-threading build (main, Jan 17 2026, 20:47:02) [Clang 17.0.0 (clang-1700.6.3.2)]'
❯ python -c 'import tink'
[1]    6318 segmentation fault  python -c 'import tink'