liblog: event_tag_map benchmark

Tested on Hikey, all services stopped, shell only access, CPUs not
locked.

$ /data/nativetest64/liblog-benchmarks/liblog-benchmarks BM_lookupEvent*
                          iterations      ns/op
Precharge: start
Precharge: stop 231
BM_lookupEventTag           10000000        153
BM_lookupEventTag_NOT       20000000        139
BM_lookupEventFormat        10000000        153

Test: run benchmarks
Bug: 31456426
Change-Id: Ice3ffa0b061d9a6b917718b2d9aedcc2348b7005