benchncnn: support running CPU and GPU tests in a single pass

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For example, on macOS:
     squeezenet cpu min =    5.14  max =    5.23  avg =    5.19  user =    5.18  sys =    0.00
     squeezenet gpu min =    1.30  max =    1.35  avg =    1.33  user =    0.13  sys =    0.05  speed_ratio =   3.90x  cpu_ratio =   3.48%

Vulkan boost the speed and reduce cpu usage.

On Android:
     squeezenet cpu min =    4.88  max =    5.06  avg =    4.98  user =   20.88  sys =    3.93
     squeezenet gpu min =   17.44  max =   19.18  avg =   18.47  user =    3.90  sys =    1.26  speed_ratio =   0.27x  cpu_ratio =  20.77%

Vulkan doesn't boost the speed but reduce cpu usage, which can be useful too.