gh-144278: Enable overriding sys.implementation's name and cache_tag when building sysmodule.c (GH-144293)

Changing the values requires forking and patching, which is intentional. Simply rebuilding from source does not change the implementation enough to justify changing these values - they would still be `cpython` and compatible with existing `.pyc` files. But people who maintain forks are better served by being able to easily override these values in a place that can be forward-ported reliably.