Add initial implementation of threads/mutexes/semaphores/cvs/spinlocks

Added initial implementation (or wrapper) of the threading library.
It's rather basic, but should work for most of the tasks.

Unfortunately, spinlock implementation relies on GCC/Clang compiler
built-ins (or in-worst-case-scenario on Win32 - InterlockExchange).
In the future, I should revisit this code and fix/reimplement some stuff
(or add support for Windows XP).

src/Platform/Dummy/Semaphore.c

@@ -0,0 +1,51 @@
+#include "Thread.h"
+
+#include <BH/Thread.h>
+
+
+BH_Semaphore *BH_SemaphoreNew(int value)
+{
+    BH_UNUSED(value);
+
+    return NULL;
+}
+
+
+void BH_SemaphoreFree(BH_Semaphore *semaphore)
+{
+    BH_UNUSED(semaphore);
+}
+
+
+int BH_SemaphorePost(BH_Semaphore *semaphore)
+{
+    BH_UNUSED(semaphore);
+
+    return BH_NOIMPL;
+}
+
+
+int BH_SemaphoreWait(BH_Semaphore *semaphore)
+{
+    BH_UNUSED(semaphore);
+
+    return BH_NOIMPL;
+}
+
+
+int BH_SemaphoreWaitTry(BH_Semaphore *semaphore)
+{
+    BH_UNUSED(semaphore);
+
+    return BH_NOIMPL;
+}
+
+
+int BH_SemaphoreWaitFor(BH_Semaphore *semaphore,
+                        uint32_t timeout)
+{
+    BH_UNUSED(semaphore);
+    BH_UNUSED(timeout);
+
+    return BH_NOIMPL;
+}