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 ltrace-0.5.3.org/Makefile.in ltrace-0.5.3/Makefile.in

--- ltrace-0.5.3.org/Makefile.in	2011-08-21 18:55:15.000000000 +0200

+++ ltrace-0.5.3/Makefile.in	2011-08-21 18:40:53.000000000 +0200

@@ -39,7 +40,7 @@

 		$(CC) $(LDFLAGS) $^ $(LIBS) -o $@

 libltrace.a: 	sysdeps/sysdep.o $(OBJ)

-		$(AR) rcv $@ $^

+		$(AR) rscv $@ $^

 $(OBJ):		sysdeps/sysdep.o