Add an output_dir override to GN.

tools/gn/ninja_binary_target_writer.cc

Index: tools/gn/ninja_binary_target_writer.cc
diff --git a/tools/gn/ninja_binary_target_writer.cc b/tools/gn/ninja_binary_target_writer.cc
index 7077b17103d83c4404157084bb51407308b1a74f..a62f8725838ad1faf7ac01c64b07db95d3c44ccb 100644
--- a/tools/gn/ninja_binary_target_writer.cc
+++ b/tools/gn/ninja_binary_target_writer.cc
@@ -780,7 +780,7 @@ void NinjaBinaryTargetWriter::WriteLinkerStuff(
     WriteLinkerFlags(optional_def_file);
     WriteLibs();
   }
-  WriteOutputExtension();
+  WriteOutputSubstitutions();
   WriteSolibs(solibs);
 }
 
@@ -847,11 +847,15 @@ void NinjaBinaryTargetWriter::WriteLibs() {
   out_ << std::endl;
 }
 
-void NinjaBinaryTargetWriter::WriteOutputExtension() {
+void NinjaBinaryTargetWriter::WriteOutputSubstitutions() {
   out_ << "  output_extension = "
        << SubstitutionWriter::GetLinkerSubstitution(
               target_, tool_, SUBSTITUTION_OUTPUT_EXTENSION);
   out_ << std::endl;
+  out_ << "  output_dir = "
+       << SubstitutionWriter::GetLinkerSubstitution(
+              target_, tool_, SUBSTITUTION_OUTPUT_DIR);
+  out_ << std::endl;
 }
 
 void NinjaBinaryTargetWriter::WriteSolibs(