libtool: Fix lto option passing for reproducible builds

If lto is enabled, we need the prefix-map variables to be passed to the linker.
Add these to the list of options libtool passes through.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2c26d2c0)
Signed-off-by: Steve Sakoman <steve@sakoman.com>