Fix build rules for gRPC proto generator plugin to work when crosscompiling

BUILD.gn

 # GRPC Chromium BUILD.gn file.
 # This file has been automatically generated from a template file.
 # Please look at the templates directory instead.
 # This file can be regenerated from the template by running
 # tools/buildgen/generate_projects.sh
 config("grpc_config") {
   include_dirs = [
     ".",
     "include/",
   ]
@@ skipping 1479 matching lines @@
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
   ]
   public_configs = [
     ":grpc_config",
   ]
 }
 
+# Only compile the plugin for the host architecture.
+if (current_toolchain == host_toolchain) {
 
 
 source_set("grpc_plugin_support") {
   sources = [
     "include/grpc++/impl/codegen/config_protobuf.h",
     "src/compiler/config.h",
     "src/compiler/cpp_generator.h",
     "src/compiler/cpp_generator_helpers.h",
     "src/compiler/csharp_generator.h",
     "src/compiler/csharp_generator_helpers.h",
@@ skipping 19 matching lines @@
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
   ]
   public_configs = [
     ":grpc_config",
   ]
 }
 
+}
 
 
+# Only compile the plugin for the host architecture.
+if (current_toolchain == host_toolchain) {
 
 executable("grpc_cpp_plugin") {
   sources = [
     "src/compiler/cpp_plugin.cc",
   ]
   deps = [
     "//third_party/protobuf:protoc_lib",
     ":grpc_plugin_support",
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//third_party/protobuf:protobuf_config",
   ]
   public_configs = [ ":grpc_config" ]
 }
 
+}
+
 
 
-