Enable CI for Loongarch64

[Fearyncess]

What does this PR do

This PR is the reimplementation of PR #2461. That will enable CI testing for loongarch64-unknown-linux-gnu.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

@Fearyncess:

I have rewrite Fearyncess#1 for Loongarch64 CI, and it works. Unfortunately, I face a big problem with the QEMU and testcases below.
Loongarch64 need a newer QEMU after 7.1 version. But before QEMU supported la64, qemu has a commit that has disabled many PRCTL subcodes 3 yrs ago. This commit will cause test_prctl failed in some cases, due to these subcodes has been disabled in newer QEMU.

BTW, I guess some commits in QEMU can change some syscalls' behavior, that brings some weird test results... -> #2418

[SteveLauC]

Please also add this target to the target support list of Nix as a tier2 target.

[SteveLauC]

BTW, I guess some commits in QEMU can change some syscalls' behavior, that brings some weird test results... -> #2418

#2418 happens without a QEMU bump, we encountered it when upgrading the Linux CI base image from Ubuntu 20.04 to 22.04🤔

[Fearyncess]

Please also add this target to the target support list of Nix as a tier2 target.

Added.

[SteveLauC]

Looks good, thanks for adding it!