ZGC: Apply workaround for buggy sem_post() in glibc < 2.21

We should apply a workaround for a bug in sem_post() in glibc < 2.21, where it's not safe to destroy the semaphore immediately after returning from sem_wait(). The reason is that sem_post() can touch the semaphore after a waiting thread have returned from sem_wait(). To avoid this race we should force the waiting thread to acquire/release the lock held by the posting thread.

For more details, see https://sourceware.org/bugzilla/show_bug.cgi?id=12674