Fix pthread benchmarks.

PauseTiming and ResumeTiming are thread-local, and unbalanced calls to
them leads to obviously bogus results like:

    Benchmark                          Time            CPU Iterations
    -----------------------------------------------------------------
    BM_pthread_exit_and_join      212600 ns    59981418 ns         12
    BM_pthread_create_and_run  212997341 ns -1569132344 ns       1000

Bug: http://b/68170209
Test: ran bionic benchmarks
Change-Id: Ia88cd6af9ff524443850d834a96cf5dd9c7f3ed9