[doit] Fix binutils extra-targets

Switch on the ARCH, not the TARGET.

Change-Id: Ieb320cc8e784eca7fbb399b498475398b2de3d2e
diff --git a/doit b/doit
index 6b5f475..74e421b 100755
--- a/doit
+++ b/doit
@@ -215,7 +215,7 @@
 
     # Building Binutils
     if [ ! -f $BINBUILDPATH/built.txt ]; then
-        case "$TARGET" in
+        case "$ARCH" in
             x86_64) EXTRA_TARGETS="--enable-targets=x86_64-pep" ;;
             *) EXTRA_TARGETS="" ;;
         esac