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: 95a87982

 ("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>