Build suffixed shared runtimes
New names for libcamlrun_shared.so and libasmrun_shared.so without the
_shared suffix and using the target triplet and runtime ID. Both ocamlc
and ocamlopt explicitly recognise `-runtime-variant _shared` and select
the correct name.
Symbolic links for libcamlrun_shared.so and libasmrun_shared.so to allow
any C programs which linked against the the output of `-output-obj` to
continue to work.