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/Win32/Semaphore.c

@@ -0,0 +1,69 @@
+#include "Thread.h"
+
+#include <BH/Thread.h>
+
+
+BH_Semaphore *BH_SemaphoreNew(int value)
+{
+    BH_Semaphore *semaphore;
+
+    /* Allocate space for mutex and initialize it */
+    semaphore = malloc(sizeof(BH_Semaphore));
+    if (semaphore)
+    {
+        semaphore->handle = CreateSemaphore(NULL, value, 0x7FFF, NULL);
+        if (!semaphore->handle)
+        {
+            free(semaphore);
+            return NULL;
+        }
+    }
+
+    return semaphore;
+}
+
+
+void BH_SemaphoreFree(BH_Semaphore *semaphore)
+{
+    CloseHandle(semaphore->handle);
+    free(semaphore);
+}
+
+
+int BH_SemaphorePost(BH_Semaphore *semaphore)
+{
+    if (!ReleaseSemaphore(semaphore->handle, 1, NULL))
+        return BH_ERROR;
+
+    return BH_OK;
+}
+
+
+int BH_SemaphoreWait(BH_Semaphore *semaphore)
+{
+    if (WaitForSingleObject(semaphore->handle, INFINITE))
+        return BH_ERROR;
+
+    return BH_OK;
+}
+
+
+int BH_SemaphoreWaitTry(BH_Semaphore *semaphore)
+{
+    if (WaitForSingleObject(semaphore->handle, 0))
+        return BH_ERROR;
+
+    return BH_OK;
+}
+
+
+int BH_SemaphoreWaitFor(BH_Semaphore *semaphore,
+                        uint32_t timeout)
+{
+    switch (WaitForSingleObject(semaphore->handle, timeout))
+    {
+    case 0: return BH_OK;
+    case WAIT_TIMEOUT: return BH_TIMEOUT;
+    default: return BH_ERROR;
+    }
+}