Concretize the result of CLZ and CTZ

[ercoppa]

SymQEMU does not provide helpers for {clz, ctz}_i{32, 64}. Meanwhile we integrate such helpers, we should at least concretize the symbolic expression of the output value. For instance, from:

tcg_gen_op3_i64(INDEX_op_clz_i64, ret, arg1, arg2);

to:

tcg_gen_op2i_i64(INDEX_op_movi_i64, tcgv_i64_expr_num(ret), 0);
tcg_gen_op3_i64(INDEX_op_clz_i64, ret, arg1, arg2);

Let me know if this fix can be a valid PR or how to improve it.