pb foreign-function interface: refine union handling for s390x (#990)

With pb, foreign calls in general use libffi, which does not provide
support for `union` arguments and results. The binding in "c/ffi.c"
approximates `union` handling by constructing a `struct` description
that is likely to be treated the same in the platform's ABI. The
approximation was wrong for s390x in the case of `union`s that contain
only floating-point fields, so add a special case for s390x. There are
surely other platforms that would need similar treatment, but s390x is
relevant because there's no native-code backend for that architecture.

Author: mflatt

c/ffi.c

@@ -145,6 +145,12 @@ ffi_type *decode_type(alloc_state *alloc, ptr type, ffi_abi abi, IBOOL *all_floa
         type = Scdr(type);
       }
 
+# ifdef __s390x__
+      /* s390x: unions containing only floats are not treated the
+         same as structs containing only floats */
+      union_all_float = 0;
+# endif
+
       if (!union_all_float)
         *all_float = 0;