If the semaphore is stored as a local variable in {CompileInParallel},
then the semaphore was sometimes deallocated too early and caused
the compilation tasks to crash. This only happens with libc-2.19,
libc-2.21 fixes the problem.

R=mlippautz@chromium.org, rossberg@chromium.org

Review-Url: https://codereview.chromium.org/2080223006
Cr-Commit-Position: refs/heads/master@{#37183}