Install ld.so to slibdir instead of rtlddir to fix cross builds

Install ld.so to slibdir instead of rtlddir to fix cross builds

When installing a cross-libc purely for linking purposes but not
runtime use, the linker shouldn't be installed in rtlddir, since
we won't actually be USING it, and thus referencing the rtlddir
path in libc.so ends up blowing up cross-compilers for no reason.

Gbp-Pq: Topic any
Gbp-Pq: Name local-rtlddir-cross.diff