scripts: correctly warn when the local tarballs directory does not exist

Warn about a missing local tarball directory, only if it was configured.
Avoid the spurious message:
Directory '${CT_LOCAL_TARBALLS_DIR}' does not exist.
Will not save downloaded tarballs to local storage.

Thanks to "Pedro I. Sanchez" <psanchez@colcan.ca> for pointing out the
issue:
http://sourceware.org/ml/crossgcc/2009-12/msg00011.html

--- binutils/ld/ldmain.c

+++ binutils/ld/ldmain.c

@@ -273,2 +273,5 @@ main (int argc, char **argv)

   link_info.emit_hash = TRUE;

+#ifndef __mips__

+  link_info.emit_gnu_hash = TRUE;

+#endif

   link_info.callbacks = &link_callbacks;