bench: Modify benchmarks for a wider range of platforms

bench: Modify benchmarks for a wider range of platforms

 - Add the new files bench.c and bench.h to implement a timer module.
 - Implement the module with clock_gettime(), mach_absolute_time(), or
   gettimeofday(), depending on a given platform.
 - Replace the time measurement code of the benchmark programs with the
   functions of the module.