Don't use spin_no_std feature

[newpavlov]

See this issue: rust-lang-nursery/lazy-static.rs#150

I think we can relatively easily replace code here with something like this:

fn is_rdrand_supported() -> bool {
    use core::arch::x86_64::__cpuid;
    use std::sync::atomic::{AtomicUsize, Ordering};
    
    static RDRAND: AtomicUsize = AtomicUsize::new(0);
    const RDRAND_FLAG: u32 = 1 << 30;
    const STATE_FLAG: usize = 1 << 1;
    const RESULT_FLAG: usize = 1 << 0;

    let flags = RDRAND.load(Ordering::Relaxed);
    if (flags & STATE_FLAG) == 0 {
        let leaf1_ecx = unsafe { __cpuid(1).ecx };
        if leaf1_ecx & RDRAND_FLAG != 0 {
            // rdrand is supported
            RDRAND.store(RESULT_FLAG | STATE_FLAG, Ordering::Release);
            true
        } else {
            // rdrand is not supported
            RDRAND.store(STATE_FLAG, Ordering::Release);
            false
        }
    } else {
        (flags & RESULT_FLAG) != 0
    }
}

In the worst-case scenario we may call CPUID several times instead of just one, but I don't think it's a big deal. Also it will remove spin crate from our dependency tree, which will help a bit with std inclusion.

cc @josephlr