Make _CLOCK_T_ system configurable

Let systems optionally provide the _CLOCK_T_ type via
<machine/_types.h>.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>

newlib/libc/include/sys/_types.h

@@ -180,7 +180,10 @@ typedef _LOCK_RECURSIVE_T _flock_t;
 typedef void *_iconv_t;
 #endif
 
+#ifndef __machine_clock_t_defined
 #define	_CLOCK_T_	unsigned long	/* clock() */
+#endif
+
 typedef	_CLOCK_T_	__clock_t;
 
 #define	_TIME_T_	long		/* time() */