Use CAMLextern in caml_ forward declarations

[jonahbeckford]

This PR changes the extern attribute in extern caml_... to be exactly what OCaml uses (CAMLextern):

https://github.com/ocaml/ocaml/blob/b0b5f92896002239491308d3f1651d6e548c109f/runtime/caml/misc.h#L123-L136

Fine Details

Without this change, on Windows 64-bit systems with OCaml bytecode you can get an obscure:

Fatal error: cannot load shared library dlldigestif_c_stubs
Reason: flexdll error: cannot relocate caml_leave_blocking_section RELOC_REL32, target is too far: FFFFFFFF2EDEC956  000000002EDEC956

because extern caml_leave_blocking_section generates the following pseudo-assembly:

CALL caml_leave_blocking_section ; this is a 32-bit relative address

which does not work safely across DLL boundaries with flexdll.

The indirection introduced by CAMLextern makes the pseudo-assembly instead:

CALL [caml_leave_blocking_section] ; this is a 64-bit indirect address

which works well with flexdll.