bpo-42161: mathmodule.c: move _PyLong_GetOne() loop invariant (GH-26391) (GH-26393)

Move _PyLong_GetZero() and _PyLong_GetOne() loop invariants outside
loops in functions:

* math.comb()
* math.gcd()
* math.lcm()
* math.perm()
(cherry picked from commit 3e7ee02327db13e4337374597cdc4458ecb9e3ad)

Co-authored-by: Victor Stinner <vstinner@python.org>