Use defined(_POSIX_THREADS) to detect pthread support (swiftlang#33609)

Co-authored-by: Ben Rimmington <[email protected]>

Author: kubamracek

include/swift/Runtime/Mutex.h

@@ -20,7 +20,11 @@
 
 #include <type_traits>
 
-#if (defined(__APPLE__) || defined(__linux__) || defined(__CYGWIN__) || defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__HAIKU__))
+#if __has_include(<unistd.h>)
+#include <unistd.h>
+#endif
+
+#if defined(_POSIX_THREADS)
 #include "swift/Runtime/MutexPThread.h"
 #elif defined(_WIN32)
 #include "swift/Runtime/MutexWin32.h"