[gn] Consolidate exec_script calls to speed up generation

runtime/vm/gypi_contents.gni

Index: runtime/vm/gypi_contents.gni
diff --git a/runtime/vm/gypi_contents.gni b/runtime/vm/gypi_contents.gni
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+# Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+_core_library_names = [
+  "async",
+  "core",
+  "collection",
+  "convert",
+  "developer",
+  "internal",
+  "isolate",
+  "math",
+  "mirrors",
+  "profiler",
+  "typed_data",
+  "vmservice",
+]
+
+_core_library_gypis = [
+  "../platform/platform_headers.gypi",
+  "../platform/platform_sources.gypi",
+  "../vm/vm_sources.gypi",
+  "../lib/async_sources.gypi",
+  "../lib/core_sources.gypi",
+  "../lib/collection_sources.gypi",
+  "../lib/convert_sources.gypi",
+  "../lib/developer_sources.gypi",
+  "../lib/internal_sources.gypi",
+  "../lib/isolate_sources.gypi",
+  "../lib/math_sources.gypi",
+  "../lib/mirrors_sources.gypi",
+  "../lib/typed_data_sources.gypi",
+  "../lib/vmservice_sources.gypi",
+  "../../sdk/lib/async/async_sources.gypi",
+  "../../sdk/lib/collection/collection_sources.gypi",
+  "../../sdk/lib/convert/convert_sources.gypi",
+  "../../sdk/lib/core/core_sources.gypi",
+  "../../sdk/lib/developer/developer_sources.gypi",
+  "../../sdk/lib/internal/internal_sources.gypi",
+  "../../sdk/lib/isolate/isolate_sources.gypi",
+  "../../sdk/lib/math/math_sources.gypi",
+  "../../sdk/lib/mirrors/mirrors_sources.gypi",
+  "../../sdk/lib/profiler/profiler_sources.gypi",
+  "../../sdk/lib/vmservice/vmservice_sources.gypi",
+  "../bin/io_sources.gypi",
+]
+
+processed_gypis = exec_script("../../tools/process_gypis.py",
+                              _core_library_names,
+                              "scope",
+                              _core_library_gypis)