Make NumCPUs() in util/benchmark.cc work better.
Make NumCPUs() in util/benchmark.cc work better.

Change-Id: If7cd0485b2d4fd7209663132e8ac63102a9dedb1
Reviewed-on: https://code-review.googlesource.com/c/36450
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2