Nightly libretro core

Mark upstream-tracked fork changes with AltirraSDL annotations (#77)

The libretro CI fixes modified three files that are synced from upstream
Windows Altirra / VirtualDub (bitmath.h, int128.h, stdafx.h). Annotate each
change with the project's "// AltirraSDL:" fork-marker convention and a
"preserve when re-syncing from upstream" note, so the next mainline merge
sees the constraint at the diff site and keeps the deviation intentionally.

Comment-only; no behavior change.