target/riscv: Drop ftemp_new

target/riscv: Drop ftemp_new

Translators are no longer required to free tcg temporaries,
therefore there's no need to record temps for later freeing.
Replace the few uses with tcg_temp_new_i64.

Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>