scripts/gcc: Canadian Cross skip -print-multi-lib log output

Attempting to ${CT_TARGET}-gcc -print-multi-lib will fail

In do_cc_core_backend, for the final compiler in a canadian cross
baremetal, warn that multi-libs cannot be determined

In do_cc_backend, for either final compiler for a canadian cross,
warn that multi-libs cannot be determined

(Plus fixed CT_PREFIX_DIR in do_cc_backend to be ${prefix})

Signed-off-by: "David Holsgrove" <david.holsgrove@xilinx.com>
Message-Id: <CAM=EW8aQDoNx-CkJHjXBoDP4iTDJ8z5hh3=KhO5UTU6rp3Pj=w@mail.gmail.com>
Patchwork-Id: 189053

 diff -ru gdb-6.8.orig/bfd/sysdep.h gdb-6.8/bfd/sysdep.h

 --- gdb-6.8.orig/bfd/sysdep.h	2007-07-03 16:26:42.000000000 +0200

 +++ gdb-6.8/bfd/sysdep.h	2011-08-21 13:33:47.000000000 +0200

 @@ -131,7 +131,7 @@

  extern PTR realloc ();

  #endif

 -#if !HAVE_DECL_STPCPY

 +#if !HAVE_DECL_STPCPY && !defined(stpcpy)

  extern char *stpcpy (char *__dest, const char *__src);

  #endif