mips: add missing pure flag to syscall entries

mips: add missing pure flag to syscall entries

* linux/mips/syscallent-n32.h ([6038], [6100], [6102], [6105], [6106],
[6108], [6109], [6178]): Add pure flag to getpid, getuid, getgid, geteuid,
getegid, getpgid, getpgrp, and gttid syscall descriptors.
* linux/mips/syscallent-n64.h ([5038], [5100], [5102], [5105], [5106],
[5108], [5109], [5178]): Likewise.
* linux/mips/syscallent-o32.h ([4020], [4024], [4047], [4049], [4050],
[4064], [4065], [4222]): Likewise.

Complements: v4.21~29 "Add pure syscall flag"