[OCaml] PR9719, PR14727: Make tests run without ocamlopt.

Previously, tests hardcoded ocamlopt and cmxa, which broke builds on
machines without ocamlopt. Instead, they now fall back to ocamlc.

As a side effect this fixes PR14727, which was caused by a crude hack
that replaced gcc with g++ everywhere in the ocamlopt native compiler
path and passes it back using -cc. Now the tests use the same
technique as META, i.e. -cclib -lstdc++. It might be more fragile
than using g++ explicitly, but it will break when the installed
package will also break, which is good.

llvm-svn: 220828