Add system_time bin command which just prints system_time().

This can be used by scripts to do verious performance measurements.
Specifically it can be used to measure the boot time since it represents
the uptime.

src/bin/system_time.cpp

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2015 Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ *
+ * Authors:
+ *		Michael Lotz, [email protected]
+ */
+
+#include <OS.h>
+
+#include <stdio.h>
+
+
+int
+main(int argc, char* argv[])
+{
+	printf("%" B_PRIdBIGTIME "\n", system_time());
+	return 0;
+}