Allow 32-bit mac->win cross builds (and as a side effect, arm64 cross builds).

After https://chromium-review.googlesource.com/c/v8/v8/+/1534635 this
works well enough automatically.

Additionally, this assert was unintentionally also hit with target_cpu =
"arm64", so this CL fixes that too.

Bug: 794838,893460
Change-Id: I32d7a67b2f818aacfaeef39a03a1107b596b9dfb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1536310
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644395}

build/config/BUILDCONFIG.gn

@@ -241,9 +241,6 @@ if (target_os == "android") {
 } else if (target_os == "win") {
   # On Windows we use the same toolchain for host and target by default.
   # Beware, win cross builds have some caveats, see docs/win_cross.md
-  # TODO(thakis): See if we want to make 32-bit builds on mac hosts work.
-  assert(host_os != "mac" || target_cpu == "x64",
-         "Mac hosts can only build 64-bit chrome/win, https://crbug.com/794838")
   if (is_clang) {
     _default_toolchain = "//build/toolchain/win:win_clang_$target_cpu"
   } else {

docs/win_cross.md

@@ -8,14 +8,15 @@ What does *not* work:
 
 * 64-bit renderer processes don't use V8 snapshots, slowing down their startup
   ([bug](https://crbug.com/803591))
-* on Mac hosts, building a 32-bit chrome ([bug](https://crbug.com/794838))
+* on Mac hosts, 32-bit builds don't use V8 snapshots either
+  ([bug](https://crbug.com/794838) has more information, but this is unlikely
+  to ever change)
 
 All other targets build fine (including `chrome`, `browser_tests`, ...).
 
-Uses of `.asm` files have been stubbed out.  As a result, some of Skia's
-software rendering fast paths are not present in cross builds, Crashpad cannot
-report crashes, and NaCl defaults to disabled and cannot be enabled in
-cross builds ([.asm bug](https://crbug.com/762167)).
+Uses of `.asm` files have been stubbed out.  As a result, Crashpad cannot
+report crashes, and NaCl defaults to disabled and cannot be enabled in cross
+builds ([.asm bug](https://crbug.com/762167)).
 
 ## .gclient setup