scripts/faddr2line: fix CROSS_COMPILE unset error

faddr2line hit var unbound error when CROSS_COMPILE isn't set since
nounset option is set in bash script.

Link: http://lkml.kernel.org/r/20171206013022.GA83929@sofia
Fixes: 95a8798254 ("scripts/faddr2line: extend usage on generic arch")
Signed-off-by: Liu Changcheng <changcheng.liu@intel.com>
Reported-by: Richard Weinberger <richard.weinberger@gmail.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/faddr2line

@@ -44,10 +44,10 @@
 set -o errexit
 set -o nounset
 
-READELF="${CROSS_COMPILE}readelf"
-ADDR2LINE="${CROSS_COMPILE}addr2line"
-SIZE="${CROSS_COMPILE}size"
-NM="${CROSS_COMPILE}nm"
+READELF="${CROSS_COMPILE:-}readelf"
+ADDR2LINE="${CROSS_COMPILE:-}addr2line"
+SIZE="${CROSS_COMPILE:-}size"
+NM="${CROSS_COMPILE:-}nm"
 
 command -v awk >/dev/null 2>&1 || die "awk isn't installed"
 command -v ${READELF} >/dev/null 2>&1 || die "readelf isn't installed"