[lsan] Use a more standard-conformant sched_yield() instead of pthread_yield().
There's no pthread_yield() on OSX (only sched_yield() and pthread_yield_np()).


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@213516 91177308-0d34-0410-b5e6-96231b3b80d8