Reuse the decodetree based disassembler from
target/loongarch/ for tcg/loongarch64/.

The generation of decode-insns.c.inc into ./libcommon.fa.p/ could
eventually result in conflict, if any other host requires the same
trick, but this is good enough for now.

Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>