[Mammoth] Add a user-space semaphore object.

lib/mammoth/src/semaphore.cpp

@@ -0,0 +1,11 @@
+#include "mammoth/semaphore.h"
+
+#include <zcall.h>
+
+#include "mammoth/debug.h"
+
+Semaphore::Semaphore() { check(ZSemaphoreCreate(&semaphore_cap_)); }
+Semaphore::~Semaphore() { check(ZCapRelease(semaphore_cap_)); }
+
+void Semaphore::Wait() { check(ZSemaphoreWait(semaphore_cap_)); }
+void Semaphore::Signal() { check(ZSemaphoreSignal(semaphore_cap_)); }