Reland "[Fuchsia] Remove the legacy gn build rules for fuchsia"

This change may require a codereview from flutter side before moving
forward. See flutter/engine#51072.

This reverts commit 283051a.

Reason for revert: Redo the change with the similar one in flutter.

After this change, the next dart -> flutter roll needs extra cares
in flutter/engine#51072. It also contains
more details about the reason why it could not be two-way
compatible.

Original change's description:
> Revert "[Fuchsia] Remove the legacy gn build rules for fuchsia"
>
> This reverts commit 094202b.
>
> Reason for revert: Break flutter, this change should happen after
> flutter being migrated to the same gn-sdk.
>
> Original change's description:
> > [Fuchsia] Remove the legacy gn build rules for fuchsia
> >
> > The updated test-scripts contains https://crrev.com/c/5341620 which
> > allows to generate fidl apis with an env var controlled location
> > rather than a hard-coded one. So the existing fuchsia gn build rules
> > in dart/sdk become obsolete and can be fully removed in favor of the
> > one in //third_party/fuchsia/gn-sdk/.
> >
> > Meanwhile the gn-sdk has been updated with
> > https://crrev.com/c/5325282 to use api-level version'ed idk but not
> > the ones in arch/.
> >
> > Cq-Include-Trybots: luci.dart.try:vm-fuchsia-release-arm64-try,vm-fuchsia-release-x64-try
> > Tested: ^^^^^
> > Bug: 40935282
> > Change-Id: I2ce958e6db1ff8221beef7b7ff953c32bb4525ba
> > Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/355283
> > Reviewed-by: Ryan Macnak <rmacnak@google.com>
> > Reviewed-by: Alexander Thomas <athom@google.com>
> > Commit-Queue: Zijie He <zijiehe@google.com>
>
> Bug: 40935282
> Change-Id: I7c455d1d362210523671c97d99ef018ede1743f4
> Cq-Include-Trybots: luci.dart.try:vm-fuchsia-release-arm64-try,vm-fuchsia-release-x64-try
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/356307
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Auto-Submit: Zijie He <zijiehe@google.com>
> Reviewed-by: Derek Xu <derekx@google.com>
> Commit-Queue: Zijie He <zijiehe@google.com>

Bug: 40935282
Change-Id: Id5a9a98013350359037b19f8506f24158c9a3120
Cq-Include-Trybots: luci.dart.try:vm-fuchsia-release-arm64-try,vm-fuchsia-release-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/356924
Commit-Queue: Zijie He <zijiehe@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

DEPS

@@ -700,7 +700,7 @@ Var("dart_root") + "/third_party/pkg/tar":
     "packages": [
       {
       "package": "chromium/fuchsia/test-scripts",
-      "version": "version:2@49064713a73ae92d8c28164938b97869afd336aa",
+      "version": "EAdD2YcYwVrhF2q_zR6xUvPkcKlPb1tJyM_6_oOc84kC",
       }
     ],
     "condition": 'download_fuchsia_deps',
@@ -711,7 +711,7 @@ Var("dart_root") + "/third_party/pkg/tar":
     "packages": [
       {
       "package": "chromium/fuchsia/gn-sdk",
-      "version": "version:2@7f1f23fce153ca079a77492d9d47d803d60b774e",
+      "version": "RgErspyYHapUO2SpcW-vo2p8yaRUMUrq0eWjRVPfQjoC",
       }
     ],
     "condition": 'download_fuchsia_deps',
@@ -836,6 +836,16 @@ hooks = [
         Var('emsdk_ver')],
     'condition': 'download_emscripten'
   },
+  {
+    'name': 'Erase arch/ from fuchsia sdk',
+    'pattern': '.',
+    'action': [
+      'rm',
+      '-rf',
+      'sdk/third_party/fuchsia/sdk/linux/arch',
+    ],
+    'condition': 'download_fuchsia_deps'
+  },
   {
     'name': 'Download Fuchsia system images',
     'pattern': '.',
@@ -847,4 +857,14 @@ hooks = [
     ],
     'condition': 'download_fuchsia_deps'
   },
+  {
+    'name': 'Generate Fuchsia GN build rules',
+    'pattern': '.',
+    'action': [
+      'python3',
+      'sdk/build/fuchsia/with_envs.py',
+      'sdk/third_party/fuchsia/test_scripts/gen_build_defs.py',
+    ],
+    'condition': 'download_fuchsia_deps'
+  },
 ]

build/config/BUILDCONFIG.gn

@@ -257,9 +257,7 @@ if (is_win) {
     "//build/config/win:winver",
   ]
   if (is_clang) {
-    _native_compiler_configs += [
-      "//build/config/win:relative_paths",
-    ]
+    _native_compiler_configs += [ "//build/config/win:relative_paths" ]
   }
 }
 if (is_posix) {
@@ -270,8 +268,8 @@ if (is_posix) {
 }
 if (is_fuchsia) {
   _native_compiler_configs += [
-    "//build/config/fuchsia:compiler",
-    "//build/config/fuchsia:runtime_library",
+    "//third_party/fuchsia/gn-sdk/src/config:compiler",
+    "//third_party/fuchsia/gn-sdk/src/config:runtime_library",
   ]
 }
 
@@ -450,8 +448,8 @@ foreach(_target_type,
         }
         if (is_fuchsia) {
           deps += [
-            "//build/config/fuchsia:runtime_library_group",
             "//build/fuchsia/config/clang:c++-runtime-deps",
+            "//third_party/fuchsia/gn-sdk/src/config:runtime_library_group",
           ]
         }
       }