[OCaml] Drop support for 3.12.1 and earlier.

[OCaml] Drop support for 3.12.1 and earlier.

In practice this means:
  * Always using -g flag.
  * Embedding -cclib -lstdc++ into the corresponding cma/cmxa file.
    This also moves -lstdc++ in a single place.
  * Using caml_named_value instead of a homegrown mechanism.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220843 91177308-0d34-0410-b5e6-96231b3b80d8