liblog: benchmark add BM_log_event_overhead_42 for a well known tag

Measure the cost of using a known test event log tag from
system/core/logcat/event.logtags:

42    answer (to life the universe etc|3)

Test: liblog_benchmarks BM_log_event_overhead_42
Bug: 36683634
Change-Id: Ibd4be82bebfcedd381c84e8078cf6d05a636aa98