thread-posix: remove the posix semaphore support

POSIX specifies an absolute time for sem_timedwait(), it would be
affected if the system time is changing, but there is not a relative
time or monotonic clock version of sem_timedwait, so we cannot gain
from POSIX semaphore any more.

An alternative way is to use sem_trywait + usleep, maybe we can
remove CONFIG_SEM_TIMEDWAIT in this way? No, because some systems
(e.g. mac os) mark the sem_xxx API as deprecated.

So maybe remove the usage of POSIX semaphore and turn to use the
pthread variant for all systems looks better.

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Message-Id: <20220222090507.2028-2-longpeng2@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/thread-posix.h

@@ -27,13 +27,9 @@ struct QemuCond {
 };
 
 struct QemuSemaphore {
-#ifndef CONFIG_SEM_TIMEDWAIT
     pthread_mutex_t lock;
     pthread_cond_t cond;
     unsigned int count;
-#else
-    sem_t sem;
-#endif
     bool initialized;
 };