Google Benchmark by default only measures time spent in the main thread.
Since our Vulkan queue implementation executes command buffers on a
separate thread, this can lead to very little "CPU" time being
reported. Use MeasureProcessCPUTime() instead to use the total CPU usage
of the process by all the threads: https://github.com/google/benchmark#cpu-timers

Bug: b/177624844
Change-Id: I071cd31095a83a5afec51120fb78e42decda2083
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52628Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Commit-Queue: Nicolas Capens <nicolascapens@google.com>