build: give the Windows main thread an 8 MiB stack (fixes #225)

[hobbo91]

Summary

Fixes #225 — on Windows, clicking Run in the configuration screen crashes
the emulator with a silent exit (STATUS_STACK_OVERFLOW, 0xc00000fd). A
regression since 0.11.0; the GUI otherwise works for configuring and saving.

Root cause

Windows reserves only ~1 MiB of stack for the main thread; Linux and macOS
default to 8 MiB. The winit event loop must run on the main thread, and the
configuration screen's Run (launcher_run → build_machine →
run_machine) builds the machine, rebuilds the host render/presentation state,
and does the first present from inside the event-loop callback — already
deep in the OS message-pump stack. That bounded-but-substantial work fits in
8 MiB but overflows 1 MiB. It crossed the 1 MiB line between 0.11 and 0.12,
which is why 0.11 was unaffected.

It reproduces only on the launcher Run path:

• Headless (--screenshot-after, unpaced) runs cleanly.
• The same bundled-AROS machine booted directly (--config, paced + audio)
  runs cleanly — the machine and run loop are fine.
• Only clicking Run — which builds deep inside the event-loop callback —
  overflows.

size_of::<Emulator>() is ~48 KiB, so this is call-stack depth on that path,
not a single oversized value on the stack.

Fix

Emit /STACK:8388608 for Windows-MSVC binary targets from build.rs, giving
the Windows main thread the same 8 MiB the other platforms already provide.
Target-gated via CARGO_CFG_TARGET_{OS,ENV} and scoped to binaries, so
Linux/macOS and library builds are untouched.

Matching the main-thread stack (rather than deferring the boot out of the
callback) is deliberate: the work is bounded and already known-good with an
8 MiB stack on the other two platforms, so this is the minimal change that
makes Windows behave the same.

Testing

• Windows MSVC release build: the binary's PE SizeOfStackReserve is now 8 MiB
  (was 1 MiB), and clicking Run boots the machine instead of crashing.
• Applies to both x86_64-pc-windows-msvc (the reporter's platform) and
  aarch64-pc-windows-msvc (where I verified). No effect on non-Windows targets.

[hobbo91]

@LinuxJedi feel free to close this if you already had a PR ready to open

[LinuxJedi]

We’ll go with your fix. It is almost identical. Many thanks for jumping on this.