win cross: Mention arm->intel cross build in docs

Bug: 1305835
Change-Id: Ic205fa72618e9644058af9b03ba569a226e2b0ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3522672
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/main@{#981047}

docs/win_cross.md

@@ -91,7 +91,20 @@ setting `DEPOT_TOOLS_WIN_TOOLCHAIN_BASE_URL` and running `gclient sync`:
 
 ## GN setup
 
-Add `target_os = "win"` to your args.gn.  Then just build, e.g.
+Add
+
+    target_os = "win"
+
+to your args.gn.
+
+If you're building on an arm host (e.g. a Mac with an Apple Silicon chip),
+you very likely also want to add
+
+    target_cpu = "x64"
+
+lest you build an arm64 chrome/win binary.
+
+Then just build, e.g.
 
     ninja -C out/gnwin base_unittests.exe