Known problems in macos version

- Codes related with procfs have not been replaced by the corresponding alternatives completely yet, such as /proc/self/maps, /proc/cpuinfo, /proc/self/stat, /proc/self/statm.
- GetDefaultLogFile() returns 'debug.log'
- Improve the performance of simulated futex; reconsider the implementation of simulated futex making it easier to migrate to other systems(such as windows)
- One test case is commented out in PopenTest.posix_popen
- dlsym may call malloc to allocate space for dlerror and causes contention profiler to deadlock at boostraping when the program is linked with libunwind. More detail please read the comment in src/bthread/mutex.cpp
- pprof does not show system functions(Known issues in pprof under macos: https://github.com/google/pprof/issues/324)