tcg/loongarch64: Support TCG_TYPE_V64

tcg/loongarch64: Support TCG_TYPE_V64

We can implement this with fld_d, fst_d for load and store,
and then use the normal v128 operations in registers.
This will improve support for guests which use v64.

Reviewed-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>