lrzsz: Use Cross AR during compile

Current code hardcodes archiver to be 'ar' from build host

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 74ed1d10)
Signed-off-by: Steve Sakoman <steve@sakoman.com>