separate out gn fake platform declaration

BUILD.gn

@@ -315,7 +315,7 @@ if (current_toolchain != "${dir_pw_toolchain}/default:default") {
 
   if (enable_fake_tests) {
     chip_build("fake_platform") {
-      toolchain = "${build_root}/toolchain/host:fake_${host_cpu}_gcc"
+      toolchain = "${build_root}/toolchain/fake:fake_${host_cpu}_gcc"
     }
   }
 

build/toolchain/fake/BUILD.gn

@@ -0,0 +1,27 @@
+# Copyright 2020 The Pigweed Authors
+# Copyright (c) 2020 Project CHIP Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import("//build_overrides/build.gni")
+import("${build_root}/toolchain/gcc_toolchain.gni")
+
+gcc_toolchain("fake_${host_cpu}_gcc") {
+  toolchain_args = {
+    current_os = host_os
+    current_os = host_os
+    current_cpu = host_cpu
+    is_clang = false
+    chip_fake_platform = true
+  }
+}

build/toolchain/host/BUILD.gn

@@ -32,15 +32,3 @@ gcc_toolchain("${host_os}_${host_cpu}_clang") {
     is_clang = true
   }
 }
-
-if (defined(chip_fake_plaform)) {
-  gcc_toolchain("fake_${host_cpu}_gcc") {
-    toolchain_args = {
-      current_os = host_os
-      current_os = host_os
-      current_cpu = host_cpu
-      is_clang = false
-      chip_fake_platform = true
-    }
-  }
-}