[3.1 blocker] Update OS X build system to include win32.S on 32-bit

The OS X build bits need updating to include win32.S in their 32-bit builds.  From an OS X build on a Mozilla trybot:
```
Undefined symbols for architecture i386:
  "_ffi_call_win32", referenced from:
      _ffi_call in libjs_static.a(ffi.o)
  "_ffi_closure_STDCALL", referenced from:
      _ffi_prep_closure_loc in libjs_static.a(ffi.o)
  "_ffi_closure_THISCALL", referenced from:
      _ffi_prep_closure_loc in libjs_static.a(ffi.o)
ld: symbol(s) not found for architecture i386
```