Macro std::is_mips64_feature_detected

macro_rules! is_mips64_feature_detected {
    ($t : tt) => { ... };
}

🔬 This is a nightly-only experimental API. (stdsimd #27731)

This is supported on non-MIPS-64 only.

Prevents compilation if is_mips64_feature_detected is used somewhere else than MIPS64 targets.