Add samplingTime mode to nanobench

tools/timer/SysTimer_posix.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "SysTimer_posix.h"
 
 static double interval_in_ms(timespec start_clock, timespec end_clock)
 {
@@ skipping 31 matching lines @@
 }
 
 double SysTimer::endWall() {
     timespec end_wall;
     if (-1 == clock_gettime(CLOCK_MONOTONIC, &end_wall)) {
         timespec none = {0, 0};
         end_wall = none;
     }
     return interval_in_ms(fWall, end_wall);
 }
+
+static timespec init_launch_time() {
+    timespec launchTime;
+    if (-1 == clock_gettime(CLOCK_MONOTONIC, &launchTime)) {
+        timespec none = {0, 0};
+        return none;
+    }
+    return launchTime;
+};
+
+double GetTimeMs() {
+    static const timespec launchTime = init_launch_time();
+    timespec now;
+    if (-1 == clock_gettime(CLOCK_MONOTONIC, &now)) {
+        return 0;
+    }
+    return interval_in_ms(launchTime, now);
+}