ci: link os_compat for replay CMake tests; fix llvm-mingw mirrors

gcc -O0 keeps unused cst_test_util.h helpers that need os_*; clang GC'd
them so only the gcc CMake step failed. Prefer ghfast/ghproxy.net for the
llvm-mingw tarball (gitclone 500 / github unreachable on the runner).

scripts/ci_windows_cross.sh

@@ -19,17 +19,25 @@ fi
 
 if [[ ! -x "$DIR/bin/x86_64-w64-mingw32-gcc" ]]; then
   echo "==> download $NAME"
+  # Prefer CN-reachable GitHub proxies; raw github.com is often unreachable.
   urls=(
+    "https://ghfast.top/https://github.com/mstorsjo/llvm-mingw/releases/download/${VER}/${NAME}.tar.xz"
+    "https://ghproxy.net/https://github.com/mstorsjo/llvm-mingw/releases/download/${VER}/${NAME}.tar.xz"
     "https://gitclone.com/github.com/mstorsjo/llvm-mingw/releases/download/${VER}/${NAME}.tar.xz"
-    "https://mirror.ghproxy.com/https://github.com/mstorsjo/llvm-mingw/releases/download/${VER}/${NAME}.tar.xz"
     "https://github.com/mstorsjo/llvm-mingw/releases/download/${VER}/${NAME}.tar.xz"
   )
   ok=0
   for u in "${urls[@]}"; do
     echo "  try $u"
-    if curl -fL --retry 2 --connect-timeout 20 --retry-delay 2 -o "$TGZ" "$u"; then
-      ok=1
-      break
+    if curl -fL --retry 2 --connect-timeout 15 --max-time 600 \
+        --retry-delay 2 -o "$TGZ" "$u"; then
+      # Reject tiny error pages that some proxies return with HTTP 200.
+      sz=$(wc -c <"$TGZ")
+      if [[ "$sz" -gt 10000000 ]]; then
+        ok=1
+        break
+      fi
+      echo "  too small ($sz bytes), next mirror"
     fi
   done
   [[ $ok -eq 1 ]] || { echo "failed to download llvm-mingw" >&2; exit 1; }