Make win/cross build work better when targeting arm64 on macOS hosts.

Remove a special case on mac hosts that was needed when is_clang wasn't
the default on Win, but is unnecessary now and gets in the way.

Bug: chromium:801780,chromium:495204
Change-Id: I8d6783c73e0c240704830a46304482e3ce94418b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1534634Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60415}

snapshot_toolchain.gni

@@ -60,9 +60,6 @@ if (v8_snapshot_toolchain == "") {
     # binaries built for the same OS, so build the snapshot with the current
     # toolchain here, too.
     v8_snapshot_toolchain = current_toolchain
-  } else if (current_os == "win" && host_os == "mac" && is_clang) {
-    # This is a mac -> win cross-compile, which is only supported w/ clang.
-    v8_snapshot_toolchain = "//build/toolchain/mac:clang_${v8_current_cpu}"
   } else if (host_cpu == "x64" &&
              (v8_current_cpu == "mips" || v8_current_cpu == "mips64")) {
     # We don't support snapshot generation for big-endian targets,
@@ -101,7 +98,7 @@ if (v8_snapshot_toolchain == "") {
     if (_cpus != "") {
       v8_snapshot_toolchain = "//build/toolchain/${host_os}:${_clang}${_cpus}"
     } else if (is_win && v8_current_cpu == "arm64") {
-      # cross compile Windows arm64 with Windows x64 toolchain.
+      # cross compile Windows arm64 with host toolchain.
       v8_snapshot_toolchain = host_toolchain
     }
   }