Revert "Enable call_empty_ffi by default"

This reverts commit ca98576.

compiler/backend/x64/x64_configScript.sml

@@ -39,7 +39,7 @@ val clos_conf = rconc (EVAL ``clos_to_bvl$default_config``)
 val bvl_conf = rconc (EVAL``bvl_to_bvi$default_config``)
 val word_to_word_conf = ``<| reg_alg:=2; col_oracle := [] |>``
 
-val x64_data_conf = ``<| tag_bits:=4; len_bits:=4; pad_bits:=2; len_size:=32; has_div:=F; has_longdiv:=T; has_fp_ops:=T; has_fp_tern:=F; call_empty_ffi:=T; gc_kind:=Simple|>``
+val x64_data_conf = ``<| tag_bits:=4; len_bits:=4; pad_bits:=2; len_size:=32; has_div:=F; has_longdiv:=T; has_fp_ops:=T; has_fp_tern:=F; call_empty_ffi:=F; gc_kind:=Simple|>``
 val x64_word_conf = ``<| bitmaps_length := 0; stack_frame_size := LN |>``
 val x64_stack_conf = ``<|jump:=T;reg_names:=x64_names|>``
 val x64_lab_conf = ``<|pos:=0;ffi_names:=NONE;labels:=LN;sec_pos_len:=[];asm_conf:=x64_config;init_clock:=5;hash_size:=104729n|>``