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

 --- glibc-2_9.orig/nptl/init.c	2007-10-15 22:25:25.000000000 +0200

 +++ glibc-2_9/nptl/init.c	2010-08-18 10:46:06.109098646 +0200

 @@ -236,7 +236,7 @@

  /* This can be set by the debugger before initialization is complete.  */

 -static bool __nptl_initial_report_events;

 +static bool __nptl_initial_report_events __attribute_used__;

  void

  __pthread_initialize_minimal_internal (void)