Fixing previously unusable 32 bit gcc toolchain

BUG=https://github.com/domokit/mojo/issues/396
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1331283005 .

build/toolchain/linux/BUILD.gn

diff --git a/build/toolchain/linux/BUILD.gn b/build/toolchain/linux/BUILD.gn
index 15ad5bd..9145284 100644
--- a/build/toolchain/linux/BUILD.gn
+++ b/build/toolchain/linux/BUILD.gn
@@ -53,7 +53,7 @@
 
 gcc_toolchain("x86") {
   cc = "${compiler_prefix}gcc"
-  cxx = "$compiler_prefix}g++"
+  cxx = "${compiler_prefix}g++"
 
   readelf = "readelf"
   nm = "nm"