Gn target to generate gen_snapshot for windows.

Adds a new target to archive gen_snapshot for windows platforms. A new
target is required because the naming convention is completely different
from the one used for linux or mac.

Bug: flutter/flutter#81855

build/archives/BUILD.gn

@@ -274,3 +274,16 @@ if (is_mac) {
     ]
   }
 }
+
+if (is_win) {
+  zip_bundle("archive_win_gen_snapshot") {
+    deps = [ "//third_party/dart/runtime/bin:gen_snapshot" ]
+    output = "$target_platform_name-$android_cpu-$flutter_runtime_mode/windows-x64.zip"
+    files = [
+      {
+        source = "$root_out_dir/gen_snapshot.exe"
+        destination = "gen_snapshot.exe"
+      },
+    ]
+  }
+}