Add an output_dir override to GN.

tools/gn/example/build/toolchain/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 toolchain("gcc") {
   tool("cc") {
     depfile = "{{output}}.d"
     command = "gcc -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
     depsformat = "gcc"
     description = "CC {{output}}"
@@ skipping 19 matching lines @@
     rspfile_content = "{{inputs}}"
     outputs = [
       "{{target_out_dir}}/{{target_output_name}}{{output_extension}}",
     ]
     default_output_extension = ".a"
     output_prefix = "lib"
   }
 
   tool("solink") {
     soname = "{{target_output_name}}{{output_extension}}"  # e.g. "libfoo.so".
+    sofile = "{{output_dir}}/$soname"
     rspfile = soname + ".rsp"
 
-    command = "g++ -shared {{ldflags}} -o $soname -Wl,-soname=$soname @$rspfile"
+    command = "g++ -shared {{ldflags}} -o $sofile -Wl,-soname=$soname @$rspfile"
     rspfile_content = "-Wl,--whole-archive {{inputs}} {{solibs}} -Wl,--no-whole-archive {{libs}}"
 
     description = "SOLINK $soname"
 
     # Use this for {{output_extension}} expansions unless a target manually
     # overrides it (in which case {{output_extension}} will be what the target
     # specifies).
     default_output_extension = ".so"
 
+    # Use this for {{output_dir}} expansions unless a target manually overrides
+    # it (in which case {{output_dir}} will be what the target specifies).
+    default_output_dir = "{{root_out_dir}}"
+
     outputs = [
-      soname,
+      sofile,
     ]
-    link_output = soname
-    depend_output = soname
+    link_output = sofile
+    depend_output = sofile
     output_prefix = "lib"
   }
 
   tool("link") {
     outfile = "{{target_output_name}}{{output_extension}}"
     rspfile = "$outfile.rsp"
     command = "g++ {{ldflags}} -o $outfile -Wl,--start-group @$rspfile {{solibs}} -Wl,--end-group {{libs}}"
     description = "LINK $outfile"
+    default_output_dir = "{{root_out_dir}}"
     rspfile_content = "{{inputs}}"
     outputs = [
       outfile,
     ]
   }
 
   tool("stamp") {
     command = "touch {{output}}"
     description = "STAMP {{output}}"
   }
 
   tool("copy") {
     command = "cp -af {{source}} {{output}}"
     description = "COPY {{source}} {{output}}"
   }
 }