toolchain: add musl libc.so to external toolchain

musl provides a /lib/libc.so file which should be integrated into the libc
package when the external toolchain with musl is used.

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@intel.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit b6a8b43dd2771d4d237256417218bed862545ef4)