Restore addr2line to cross tools

Addr2line is not required for the build, and a per-arch binary is no
longer required with the switch to the ELF Tool Chain. However, building
these tools during the cross tools stage can be useful for developers
who cross build HEAD from stable/10, and adds very little to the build
time.

Reviewed by: ian, imp
Differential Revision: https://reviews.freebsd.org/D1583