toolchain-scripts: compatibility with unbound variable protection

Fixed an error when Bash's unbound variable protection is enabled (set -u) and variable "LD_LIBRARY_PATH" does not exist.

Signed-off-by: Jan Kircher <openembedded@hetsh.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 85685370)
Signed-off-by: Steve Sakoman <steve@sakoman.com>