pthread: implement sem_destroy

Note that we don't allocate memory in sem_init: we are using placement new to
just construct the object over an already existing memory location. Therefore,
all we need to do is release our unique_ptr

Thanks Pawel for noticing we need to release memory of the internal semaphore

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>

libc/sem.cc

@@ -29,6 +29,12 @@ int sem_init(sem_t* s, int pshared, unsigned val)
     return 0;
 }
 
+int sem_destroy(sem_t *s)
+{
+    from_libc(s).~indirect_semaphore();
+    return 0;
+}
+
 int sem_post(sem_t* s)
 {
     from_libc(s)->post();