Add C++11 thread support

[thrimbor]

Enables support for C++11 threads, based on the Win32 threads backend. Depends on https://github.com/XboxDev/nxdk/pull/274.

Minor modifications are done to adjust libc++ for nxdk-specific details, such as condition variables being a different size than on Windows. It also uses DestroyConditionVariable, which is an nxdk-specific addition to prevent leaking event handles (Windows Vista and newer have kernel mechanisms that allow for an implementation of condition variables that doesn't require the creation of events, so this is not an issue there).

It can be tested with something like this (don't forget NXDK_CXX=y ofc):

  std::thread t([] {
    debugPrint("hello!\n");
    Sleep(5000);
    debugPrint("done!\n");
  });

  t.join();
  debugPrint("thread done\n");

[thrimbor]

Just pushed a small update to fix compatibility with the DestroyConditionVariable -> UninitializeConditionVariable rename.

[JayFoxRox]

Tests by others (maintainers, contributors and users alike) would be welcome. I plan to merge this soon; most of the implementation has been part of https://github.com/XboxDev/nxdk/pull/274, so we mostly need a quick review and someone to say "Works" (after testing).

[GXTX]

Tested this today after writing some bad code, seems to work as expected. See bad code

[dracc]

Tested with this code: https://hastebin.com/yebiwokize.cpp Got this result:

Same code works fine without the changes of this pull request.

[dracc]

Tested with the thrimborfixes branch of NevolutionX, works as expected. :+1: