linker: Include the .ARM.extab section for Rust

This is a little complicated.  We currently just jump directly to rust's
panic, so we aren't likely to actually need these unwind tables.  But, they
would be needed if we ever reall paniced.  As of the time of this change,
the tables seem to just be 24 bytes of ROM space.

Signed-off-by: David Brown <david.brown@linaro.org>