[3.14] gh-140431: Fix GC crash due to partially initialized coroutines (gh-140470) (gh-140504)

The `make_gen()` function creates and tracks generator/coro objects, but
doesn't fully initialize all the fields. At a minimum, we need to
initialize all the fields that may be accessed by gen_traverse because
the call to `compute_cr_origin()` can trigger a GC.
(cherry picked from commit 574405c19e9b5de0504be46a3925027ded4495ae)

Co-authored-by: Sam Gross <colesbury@gmail.com>