build.rs: Questionable check `target_os == "loongarch64"` in musl_v1_2_3 check

[neuschaefer]

While browsing the libc crate source code, I noticed this part:

    // loongarch64 and ohos have already updated
    if musl_v1_2_3 || target_os == "loongarch64" || target_env == "ohos" {
        // FIXME(musl): enable time64 api as well
        set_cfg("musl_v1_2_3");
    }

This is suspicious because it should probably be target_arch, not target_os.

[tgross35]

Oh boy, you're right. This was introduced at #4443 but must not have any effect since those tests aren't failing.

Please feel free to submit a PR if you're already looking at the code, otherwise I'll try to put one up at some point.

[neuschaefer]

I'll leave it up to you, since I'm not looking that deeply