simpleperf: combine mapped buffer used for the same event and cpu.

simpleperf: combine mapped buffer used for the same event and cpu.

Non root users have a size limit of buffers used for profiling.
By combining buffers used for the same event and cpu, we can
reduce buffer cost when monitoring multiple threads, thus
avoid mmap() failures.

Increase default value for -m option for non system wide profiling.

Bug: 28911532
Change-Id: I91148061eb54840c144cf72e7bb901e7b74897ec
Test: run simpleperf_unit_test.