[gn] Move build to gypfiles

build/mac/asan.gyp

-# Copyright 2015 the V8 project authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-{
-   'targets': [
-     {
-       'target_name': 'asan_dynamic_runtime',
-       'toolsets': ['target', 'host'],
-       'type': 'none',
-       'variables': {
-         # Every target is going to depend on asan_dynamic_runtime, so allow
-         # this one to depend on itself.
-         'prune_self_dependency': 1,
-         # Path is relative to this GYP file.
-         'asan_rtl_mask_path':
-             '../../third_party/llvm-build/Release+Asserts/lib/clang/*/lib/darwin',
-         'asan_osx_dynamic':
-             '<(asan_rtl_mask_path)/libclang_rt.asan_osx_dynamic.dylib',
-       },
-       'copies': [
-         {
-           'destination': '<(PRODUCT_DIR)',
-           'files': [
-             '<!(/bin/ls <(asan_osx_dynamic))',
-           ],
-         },
-       ],
-     },
-   ],
-}