toolchain-external: install libatomic

It's required in some 32-bit architectures for the extended (64-bit)
atomic operations, like __sync_add_and_fetch_8.
These arches are at least: i386, mips & mipsel.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>