Make _CLOCKID_T_ system configurable

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

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

newlib/libc/include/sys/_types.h

@@ -193,7 +193,10 @@ typedef	_CLOCK_T_	__clock_t;
 #endif
 typedef	_TIME_T_	__time_t;
 
+#ifndef __machine_clockid_t_defined
 #define	_CLOCKID_T_ 	unsigned long
+#endif
+
 typedef	_CLOCKID_T_	__clockid_t;
 
 #define	_TIMER_T_	unsigned long