toolchain: add musl libc.so to external toolchain

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 <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Acked-by: Jo-Philipp Wich <[email protected]>