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

 --- uClibc-0.9.30.orig/extra/scripts/install_headers.sh	2008-10-10 15:50:38.000000000 +0200

 +++ uClibc-0.9.30/extra/scripts/install_headers.sh	2011-08-21 09:49:03.000000000 +0200

 @@ -33,7 +33,7 @@

  (

  # We must cd, or else we'll prepend "$1" to filenames!

  cd "$1" || exit 1

 -find ! -name '.' -a ! -path '*/.*' | sed -e 's/^\.\///' -e '/^config\//d' \

 +find . ! -name '.' -a ! -path '*/.*' | sed -e 's/^\.\///' -e '/^config\//d' \

  	-e '/^config$/d'

  ) | \

  (