Use clang as assembler on Mac OS X if available.

Mac OS X ships with a really old version of as, which does not support
the CFI directives. LLVM itself provides an assembler that is certainly
way more up to date, but currently there's no dedicated frontend, so we
simply use clang in this case.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13226 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
[amd64] Avoid unbalanced CFA offsets.

This fixes the following warnings generated by ld on Mac OS X:

- ld: warning: could not create compact unwind for _caml_raise_exn: stack subq instruction is too different from dwarf stack size
- ld: warning: could not create compact unwind for _caml_raise_exception: stack subq instruction is too different from dwarf stack size

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13227 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02