fix define for YIELDING

The intent seem to be to define this in one way, but previously it was
then immediately overriding it for various architectures, causing a
compiler warning.

Author: vtjnash

common.h

@@ -364,15 +364,15 @@ typedef int blasint;
 
 #if defined(OS_SUNOS)
 #define YIELDING	thr_yield()
-#endif
 
-#if defined(OS_WINDOWS)
+#elif defined(OS_WINDOWS)
 #if defined(_MSC_VER) && !defined(__clang__)
 #define YIELDING    YieldProcessor()
 #else
 #define YIELDING	SwitchToThread()
 #endif
-#endif
+
+#else
 
 #if defined(ARMV7) || defined(ARMV6) || defined(ARMV8) || defined(ARMV5)
 #define YIELDING        __asm__ __volatile__ ("nop;nop;nop;nop;nop;nop;nop;nop; \n");
@@ -406,6 +406,7 @@ typedef int blasint;
 #ifndef YIELDING
 #define YIELDING	sched_yield()
 #endif
+#endif
 
 /***
 To alloc job_t on heap or stack.