Fix OCaml bindings crash, PR8847.

See http://caml.inria.fr/mantis/view.php?id=4166
If we call only external functions from a module, then its 'let _' bindings
don't get executed, which means that the exceptions don't get registered for use
in the C code.
This in turn causes llvm_raise to call raise_with_arg() with a NULL pointer and
cause a segmentation fault.

The workaround is to declare all 'external' functions as 'val' in these .mli
files.

Also added a separate testcase (the testcase must call only external functions
for the bug to occur).

llvm-svn: 122497