add caml_get_header to runtime5

[riaqn]

Forward port the runtime part of ocaml-flambda/ocaml-jst@5be3cb8

Note that as of now, runtime5 doesn't have stack allocation, so lib-obj/get_header.ml will fail as some object are on heap (while expected to be on stack)

[mshinwell]

@riaqn can you fix the test case so that it passes when stack allocation is disabled too? There are some examples in typing-local/ as to how to do this, using the stack-allocation ocamltest predicate.

[riaqn]

I suggest postpone this PR until proper local allocation is supported in runtime; because currently is_stack cannot be defined, becuase I don't know what would be the color for stack objects. And is_stack is a major motivation for get_header.