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

templates/BUILD.gn.template

 %YAML 1.2
 --- |
   # 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 = [
       ".",
@@ skipping 56 matching lines @@
       or f.endswith("load_balancer_api.c"))
 
   def wanted_lib(lib):
     wanted_libs = ("gpr", "grpc", "grpc_unsecure", "grpc++", "grpc++_unsecure",
                    "grpc_cronet", "grpc_plugin_support")
     return lib.build in ("all", "protoc") and lib.get("name", "") in wanted_libs
 
   def wanted_binary(tgt):
     wanted_binaries = ("grpc_cpp_plugin",)
     return tgt.build == "protoc" and tgt.get("name", "") in wanted_binaries
+
+  def only_on_host_toolchain(tgt):
+    return tgt.get("name", "") in ("grpc_plugin_support", "grpc_cpp_plugin")
   %>
 
   % for lib in libs:
   % if wanted_lib(lib):
+  % if only_on_host_toolchain(lib):
+  # Only compile the plugin for the host architecture.
+  if (current_toolchain == host_toolchain) {
   ${cc_library(lib)}
+  }
+  % else:
+  ${cc_library(lib)}
+  % endif
   % endif
   % endfor
 
 
   % for tgt in targets:
   % if wanted_binary(tgt):
+  % if only_on_host_toolchain(tgt):
+  # Only compile the plugin for the host architecture.
+  if (current_toolchain == host_toolchain) {
   ${cc_binary(tgt)}
+  }
+  % else:
+  ${cc_binary(tgt)}
+  % endif
   % endif
   % endfor
 
   <%def name="cc_library(lib)">
   <%
     lib_hdrs = lib.get("headers", [])
     hdrs = [h for h in lib_hdrs if not uses_nanopb_or_protofull(h)]
     srcs = [s for s in lib.src if not uses_nanopb_or_protofull(s)]
   %>
   source_set("${lib.name}") {
@@ skipping 41 matching lines @@
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [
       "//build/config/compiler:no_chromium_code",
   % for config in get_extra_configs(tgt):
       "${config}",
   % endfor
     ]
     public_configs = [ ":grpc_config" ]
   }
   </%def>