[3.12] gh-110820: Disable test_signal.test_stress_modifying_handlers on macOS (GH-112834)

* gh-110820: Make sure processor specific defines are correct for Universal 2 build on macOS (#112828)

A number of processor specific defines are different for x86-64 and
arm64, and need to be adjusted in pymacconfig.h.

(cherry picked from commit 15a80b15af9a0b0ebe6bd538a1919712ce7d4ef9)