toolchain/toolchain-external: libatomic should also be copied for musl toolchains

libatomic, like libgcc_s, is provided by gcc, so there is no reason to
copy it over only for the glibc and uclibc cases, it should also be
copied for the musl case. Without this, a program linked with
libatomic on a musl system will fail to run due to the missing
library.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>