[PATCH v2] linux: Fix stat layout divergence under pre-POSIX.1-2008 macros [BZ #32119]

[Matthias Goergens]

I ran tst-stat-layout-time64 against v2 on two configurations:

- x86_64-linux-gnu, gcc 16.2, native build: PASS
- arm-linux-gnueabihf, gcc 14.2 (Debian cross compiler on a Raspberry
  Pi 4): PASS

The armhf run covers the __stat64_t64 unification this version adds
after the v1 CI regression.  One note: the Makefile hunk needed manual
application on current master (04e750e7) — the tst-sched-consts context
above it has drifted — so v3 may want a rebase.

Tested-by: Matthias Goergens <matthias.goergens@gmail.com>