Implement clock()

Newlib uses _times_r() in clock().  The problem is that the _times_r()
clock frequency is defined by sysconf(_SC_CLK_TCK).  The clock frequency
of clock() is the constant CLOCKS_PER_SEC.

FreeBSD uses getrusage() for clock().  Since RTEMS has only one process,
the implementation can be simplified.

Update #3121.

cpukit/libcsupport/Makefile.am

@@ -66,6 +66,7 @@ SYSTEM_CALL_C_FILES = src/open.c src/close.c src/read.c src/write.c \
     src/fcntl.c src/fpathconf.c src/getdents.c src/fsync.c src/fdatasync.c \
     src/pipe.c src/dup.c src/dup2.c src/symlink.c src/readlink.c \
     src/chroot.c src/sync.c src/_rename_r.c src/statvfs.c src/utimes.c src/lchown.c
+SYSTEM_CALL_C_FILES += src/clock.c
 
 ## Until sys/uio.h is moved to libcsupport, we have to have networking
 ## enabled to compile these.  Hopefully this is a temporary situation.

cpukit/libcsupport/src/clock.c

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2017 embedded brains GmbH.  All rights reserved.
+ *
+ *  embedded brains GmbH
+ *  Dornierstr. 4
+ *  82178 Puchheim
+ *  Germany
+ *  <rtems@embedded-brains.de>
+ *
+ * The license and distribution terms for this file may be
+ * found in the file LICENSE in this distribution or at
+ * http://www.rtems.org/license/LICENSE.
+ */
+
+#if HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <time.h>
+
+#include <rtems/score/basedefs.h>
+#include <rtems/score/timecounter.h>
+
+RTEMS_STATIC_ASSERT( CLOCKS_PER_SEC == 1000000, clocks_per_sec );
+
+clock_t clock( void )
+{
+  struct timeval tv;
+
+  _Timecounter_Microuptime( &tv );
+
+  return (clock_t) (tv.tv_sec - 1) * CLOCKS_PER_SEC + tv.tv_usec;
+}