Maintain runtime of each benchmark at ~10 seconds

The idea to run benchmarks for a constant number of iterations is
problematic.  While the benchmarks may run for 10 seconds on x86_64,
they could run for about 30 seconds on powerpc and worse, over 3
minutes on arm.  Besides that, adding a new benchmark is cumbersome
since one needs to find out the number of iterations needed for a
sufficient runtime.

A better idea would be to run each benchmark for a specific amount of
time.  This patch does just that.  The run time defaults to 10 seconds
and it is configurable at command line:

  make BENCH_DURATION=5 bench

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