PR#6082: on x86-32, enforce 16-byte stack alignment for compatibility
with recent GCC and Clang.  Win32/MSVC keeps 4-byte stack alignment.


git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14943 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02