Fix typo in gn ccache support for linux:x86

build/toolchain/linux/BUILD.gn

 # Copyright 2013 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.
 
 import("//build/config/sysroot.gni")
 import("//build/toolchain/ccache.gni")
 import("//build/toolchain/clang.gni")
 import("//build/toolchain/gcc_toolchain.gni")
 import("//build/toolchain/goma.gni")
 
@@ skipping 35 matching lines @@
   ar = "ar"
   ld = cxx
 
   toolchain_cpu = "x86"
   toolchain_os = "linux"
   is_clang = true
 }
 
 gcc_toolchain("x86") {
   cc = "${compiler_prefix}gcc"
-  cxx = "$compiler_prefix}g++"
+  cxx = "${compiler_prefix}g++"
 
   readelf = "readelf"
   nm = "nm"
   ar = "ar"
   ld = cxx
 
   toolchain_cpu = "x86"
   toolchain_os = "linux"
   is_clang = false
 }
@@ skipping 38 matching lines @@
   cxx = "mipsel-linux-gnu-g++"
   ar = "mipsel-linux-gnu-ar"
   ld = cxx
   readelf = "mipsel-linux-gnu-readelf"
   nm = "mipsel-linux-gnu-nm"
 
   toolchain_cpu = "mipsel"
   toolchain_os = "linux"
   is_clang = false
 }