LibPthread: Implement named semaphores

Note that as part of this commit semaphore.cpp is excluded from the
DynamicLoader, as the dynamic loader does not build with pthread.cpp
which semaphore.cpp uses.

Userland/Libraries/LibC/semaphore.h

@@ -14,6 +14,7 @@
 __BEGIN_DECLS
 
 #define SEM_FLAG_PROCESS_SHARED (1 << 0)
+#define SEM_FLAG_NAMED (1 << 1)
 typedef struct {
     uint32_t magic;
     uint32_t value;