移植 HUSTOJ judge_clinet 到 LoongArch64

HUSTOJ 是一款流行的OJ系统，跨平台、易安装、有题库、二次开发门槛低。具有多个分支，我参与移植的是 zhblue/hustoj 张老师维护的分支。

大概记录一下移植的过程。目前该 PR 已经合并到了 master 上，当前实测是支持 C/C++ 判题，后续根据上游支持情况继续适配其他语言。

参考了 [原]给HUSTOJ用户提供的源码阅读与修改建议中关于 judge_client 的代码解读，大概把握了判题机的处理思路。

1、关于 LoongArch64 的系统调用寄存器的位置信息。参考了nolibc/arch-loongarch.h。

// https://github.com/torvalds/linux/blob/master/tools/include/nolibc/arch-loongarch.h
/* Syscalls for LoongArch :
 *   - stack is 16-byte aligned
 *   - syscall number is passed in a7
 *   - arguments are in a0, a1, a2, a3, a4, a5
 *   - the system call is performed by calling "syscall 0"
 *   - syscall return comes in a0
 *   - the arguments are cast to long and assigned into the target
 *     registers which are then simply passed as registers to the asm code,
 *     so that we don't have to experience issues with register constraints.
 */