Workaround for github issue #90

The current implementation using sched_yield() does not reliably give
other threads a chance to run. The problem is demonstrated by the poor
performance of the following test case:

    out/ppc.release/cctest test-api/RegExpStringModification

This test creates a second thread which needs to acquire a pthread
mutex owned by the main thread. The main thread continually
releases/reacquires the mutex around a call to Thread::YieldCPU (see
RuntimePreempt), but the second thread still experiences long wait
times.

This commit replaces sched_yield with Sleep(0) for PPC linux.