liblog: add benchmark tests for not-printed message

Add a BM_log_verbose_overhead test that measure the overhead of
writing a log message that will not be printed due to verbosity
levels.  This was optimized in R to not format the log messages before
checking if they will be logged.

Results with R:
BM_log_verbose_overhead_null_mean          159 ns     159 ns    100
BM_log_verbose_overhead_null_median        159 ns     159 ns    100
BM_log_verbose_overhead_null_stddev        0.568 ns   0.464 ns  100
Results with Q:
M_log_verbose_overhead_null_mean           601 ns     600 ns    100
BM_log_verbose_overhead_null_median        601 ns     600 ns    100
BM_log_verbose_overhead_null_stddev        2.40 ns    2.23 ns   100

This benchmark shows liblog's overhead when not printing a log message
to be nearly ~4x faster than with Q.

Test: run this benchmark
Change-Id: I09a60e61bf064330bf15a9d0a946acf16e777a6d