use YieldProcessor on win32 for both C and C++ for consistency (pr #1215)

Author: daanx

include/mimalloc/atomic.h

@@ -358,14 +358,9 @@ typedef _Atomic(uintptr_t) mi_atomic_guard_t;
 // Yield
 // ----------------------------------------------------------------------
 
-#if defined(__cplusplus)
-#include <thread>
-static inline void mi_atomic_yield(void) {
-  std::this_thread::yield();
-}
-#elif defined(_WIN32)
+#if defined(_WIN32)
 static inline void mi_atomic_yield(void) {
-  YieldProcessor();
+  YieldProcessor();  // see issue #1215 and #1225 why this is preferred over __yield or SwitchToThread
 }
 #elif defined(__SSE2__)
 #include <emmintrin.h>
@@ -406,7 +401,6 @@ static inline void mi_atomic_yield(void) {
 #endif
 #endif
 #elif defined(__sun)
-// Fallback for other archs
 #include <synch.h>
 static inline void mi_atomic_yield(void) {
   smt_pause();
@@ -416,6 +410,12 @@ static inline void mi_atomic_yield(void) {
 static inline void mi_atomic_yield(void) {
   sched_yield();
 }
+// Fallback for other archs
+#elif defined(__cplusplus)  
+#include <thread>
+static inline void mi_atomic_yield(void) {
+  std::this_thread::yield();
+}
 #else
 #include <unistd.h>
 static inline void mi_atomic_yield(void) {