[3.12] gh-105091: stable_abi.py: Remove "Unixy" check from --all on other platforms (GH-105092) (GH-105133)

gh-105091: stable_abi.py: Remove "Unixy" check from --all on other platforms (GH-105092)
(cherry picked from commit 0656d23d82cd5b88e578a26c65dd4a64414c833b)

Co-authored-by: Petr Viktorin <encukou@gmail.com>