Added a bionic systrace class and tracing to pthread_mutex.cpp.

bionic_systrace.h contains an implementation of tracing that
can be used with systrace.py and its associated viewer.  pthread_mutex
now uses this tracing to track pthread_mutex contention, which can be
enabled by using the "bionic" command line option to systrace.

Bug: 15116468
Change-Id: I30ed5b377c91ca4c36568a0e647ddf95d4e4a61a