Enable cross-building V8 Fuchsia port on Mac.

Add necessary dependencies and rules to produce a functional
Fuchsia d8 package from a standalone V8 build.

R=adamk
BUG=

Change-Id: If81cc9fc37822cda47bb1fe1846b9519c8fcbf40
Reviewed-on: https://chromium-review.googlesource.com/c/1226414
Commit-Queue: Dimitri Glazkov <dglazkov@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56425}

BUILD.gn

@@ -3443,6 +3443,24 @@ group("v8_archive") {
   ]
 }
 
+# TODO(dglazkov): Remove the "!build_with_chromium" condition once this clause
+# is removed from Chromium.
+if (is_fuchsia && !build_with_chromium) {
+  import("//build/config/fuchsia/rules.gni")
+
+  fuchsia_package("d8_fuchsia_pkg") {
+    testonly = true
+    binary = ":d8"
+    package_name_override = "d8"
+  }
+
+  fuchsia_package_runner("d8_fuchsia") {
+    testonly = true
+    package = ":d8_fuchsia_pkg"
+    package_name_override = "d8"
+  }
+}
+
 group("v8_fuzzers") {
   testonly = true
   data_deps = [

DEPS

@@ -70,6 +70,16 @@ deps = {
       'condition': 'host_os == "linux" and checkout_fuchsia',
       'dep_type': 'cipd',
   },
+  'v8/third_party/qemu-mac-x64': {
+      'packages': [
+          {
+              'package': 'fuchsia/qemu/mac-amd64',
+              'version': '2d3358ae9a569b2d4a474f498b32b202a152134f'
+          },
+      ],
+      'condition': 'host_os == "mac" and checkout_fuchsia',
+      'dep_type': 'cipd',
+  },
   'v8/tools/clang':
     Var('chromium_url') + '/chromium/src/tools/clang.git' + '@' + '7792d28b069af6dd3a86d1ba83b7f5c4ede605dc',
   'v8/tools/luci-go':
@@ -376,6 +386,23 @@ hooks = [
       'v8/build/fuchsia/update_sdk.py',
     ],
   },
+  {
+    # Mac doesn't use lld so it's not included in the default clang bundle
+    # there. However, lld is need in Fuchsia cross builds, so
+    # download it there.
+    # Should run after the clang hook.
+    'name': 'lld/mac',
+    'pattern': '.',
+    'condition': 'host_os == "mac" and checkout_fuchsia',
+    'action': ['python', 'v8/tools/clang/scripts/download_lld_mac.py'],
+  },
+  {
+      # Mac does not have llvm-objdump, download it for cross builds in Fuchsia.
+    'name': 'llvm-objdump',
+    'pattern': '.',
+    'condition': 'host_os == "mac" and checkout_fuchsia',
+    'action': ['python', 'v8/tools/clang/scripts/download_objdump.py'],
+  },
   {
     'name': 'mips_toolchain',
     'pattern': '.',