Fix C output for modern C initiative

GCC 14 on aarch64 rejects the C code written by GHC with this kind of
error:

   error: assignment to ‘ffi_arg’ {aka ‘long unsigned int’} from ‘HsPtr’ {aka ‘void *’} makes integer from pointer without a cast [-Wint-conversion]
         68 | *(ffi_arg*)resp = cret;
            |                 ^

Add the correct cast.

For more information on this see:
https://fedoraproject.org/wiki/Changes/PortingToModernC



Tested-by: Richard W.M. Jones <rjones@redhat.com>

compiler/GHC/HsToCore/Foreign/C.hs

@@ -560,7 +560,7 @@ mkFExportCBits dflags c_nm maybe_target arg_htys res_hty is_IO_res_ty cc
      ,   ppUnless res_hty_is_unit $
          if libffi
                   then char '*' <> parens (ffi_cResType <> char '*') <>
-                       text "resp = cret;"
+                       text "resp = " <> parens ffi_cResType <> text "cret;"
                   else text "return cret;"
      , rbrace
      ]