Replaced nanosleep and sched_yield with STL routines

[jrmadsen]

• nanosleep and sched_yield are specific to UNIX
• closes error when compiled using gcc-aarch64-none-elf #3816 (theoretically)
• KOKKOS_ENABLE_STDTHREAD is probably old pre-C++11 code. It isn't defined anywhere so it was removed

Benchmarked (via QuickBench:

      auto start = std::chrono::high_resolution_clock::now();
      std::this_thread::yield();
      std::this_thread::sleep_until(start + std::chrono::nanoseconds(c * 1000));

vs.

      std::this_thread::yield();
      std::this_thread::sleep_for(std::chrono::microseconds(c));

and found the latter to be more efficient.

@crtrott Can you take a look at HostBarrier::impl_backoff_wait_until_equal as a whole? There is a weird #if !defined(_WIN32) and the #else for Windows doesn't seem to implement the same algorithm at all.

[dalg24]

LGTM regardless of whether we resolve the #if defined(_WIN32)

[crtrott]

THe else doesn't implement the same algorithm due to the use of unix stuff in the other. Essentially it should be doing the same as Linux now if we can use non-unix specific stuff for it.

[crtrott]

Jonathan do you want to address the windows stuff? I think its partly just removing a the alternative code path or?