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

 --- a/bfd/elf32-arm.c

 +++ b/bfd/elf32-arm.c

 @@ -12408,7 +12408,8 @@ elf32_arm_check_relocs (bfd *abfd, struc

  	  /* If the symbol is a function that doesn't bind locally,

  	     this relocation will need a PLT entry.  */

 -	  root_plt->refcount += 1;

 +	  if (root_plt->refcount != -1)

 +	    root_plt->refcount += 1;

  	  if (!call_reloc_p)

  	    arm_plt->noncall_refcount++;