add support for targeting glibc 2.31

Previous release issue: #3098

[glibc 2.31 was released](https://sourceware.org/ml/libc-announce/2020/msg00001.html). Time to follow the [update procedure](https://github.com/ziglang/zig/wiki/Updating-libc#glibc).

In master branch this is done in stage1. However I will be looking into self-hosting building glibc (#4313) before solving this issue.