target/loongarch: Add preldx instruction

Resolve the issue of starting the Loongnix 20.5[1] system failure.

Logs:
    Loading Linux 4.19.0-19-loongson-3 ...
    Loading initial ramdisk ...
    PROGRESS CODE: V02010004 I0
    PROGRESS CODE: V03101019 I0
    Error: unknown opcode. 90000000003a3e6c: 0x382c6d82

[1] http://pkg.loongnix.cn/loongnix/isos/Loongnix-20.5/Loongnix-20.5.cartoon.gui.loongarch64.en.qcow2

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20230905123910.3052023-1-gaosong@loongson.cn>
3 files changed