organize std lib concurrency primitives and add RwLock

* move concurrency primitives that always operate on kernel threads to
   the std.Thread namespace
 * remove std.SpinLock. Nobody should use this in a non-freestanding
   environment; the other primitives are always preferable. In
   freestanding, it will be necessary to put custom spin logic in there,
   so there are no use cases for a std lib version.
 * move some std lib files to the top level fields convention
 * add std.Thread.spinLoopHint
 * add std.Thread.Condition
 * add std.Thread.Semaphore
 * new implementation of std.Thread.Mutex for Windows and non-pthreads Linux
 * add std.Thread.RwLock

Implementations provided by @kprotty

lib/std/c/emscripten.zig

@@ -9,5 +9,8 @@ pub const pthread_mutex_t = extern struct {
 pub const pthread_cond_t = extern struct {
     size: [__SIZEOF_PTHREAD_COND_T]u8 align(@alignOf(usize)) = [_]u8{0} ** __SIZEOF_PTHREAD_COND_T,
 };
+pub const pthread_rwlock_t = extern struct {
+    size: [32]u8 align(4) = [_]u8{0} ** 32,
+};
 const __SIZEOF_PTHREAD_COND_T = 48;
 const __SIZEOF_PTHREAD_MUTEX_T = 28;