add regexp_benchmark target

Tested on Linux with the following command lines:

$ bazel build //...
$ bazel test //...
$ bazel run //:regexp_benchmark

Change-Id: I5872f0dd899c1c1c1f10c9d0f2fe13901b2c8e91
Reviewed-on: https://code-review.googlesource.com/4370
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed