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Sometimes I can see that 'perf record' piped with 'perf inject' take a
long time processing build-ids.

So introduce a inject-build-id benchmark to the internals benchmark
suite to measure its overhead regularly.

It runs the 'perf inject' command internally and feeds the given number
of synthesized events (MMAP2 + SAMPLE basically).

  Usage: perf bench internals inject-build-id <options>

    -i, --iterations <n>  Number of iterations used to compute average (default: 100)
    -m, --nr-mmaps <n>    Number of mmap events for each iteration (default: 100)
    -n, --nr-samples <n>  Number of sample events per mmap event (default: 100)
    -v, --verbose         be more verbose (show iteration count, DSO name, etc)

By default, it measures average processing time of 100 MMAP2 events
and 10000 SAMPLE events.  Below is a result on my laptop.

  $ perf bench internals inject-build-id
  # Running 'internals/inject-build-id' benchmark:
    Average build-id injection to...