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/Spinlock.c

@@ -0,0 +1,50 @@
+#include <BH/Thread.h>
+
+
+#if defined(__clang__) || defined(__GNUC__)
+/* Using built-ins */
+#elif defined(_WIN32)
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#endif
+
+
+void BH_SpinlockLock(int *lock)
+{
+#if defined(__clang__) || defined(__GNUC__)
+    while (__sync_lock_test_and_set(lock, 1));
+#elif defined(_WIN32)
+    while (InterlockedExchange(lock, 1));
+#else
+    #error "Spinlocks are not supported"
+#endif
+}
+
+
+int BH_SpinlockLockTry(int *lock)
+{
+#if defined(__clang__) || defined(__GNUC__)
+    if (__sync_lock_test_and_set(lock, 1))
+        return BH_ERROR;
+    return BH_OK;
+#elif defined(_WIN32)
+    if (InterlockedExchange(lock, 1))
+        return BH_ERROR;
+    return BH_OK;
+#else
+    #error "Spinlocks are not supported"
+    return BH_NOIMPL;
+#endif
+}
+
+
+void BH_SpinlockUnlock(int *lock)
+{
+#if defined(__clang__) || defined(__GNUC__)
+    __sync_lock_release(lock);
+#elif defined(_WIN32)
+    InterlockedExchange(lock, 0);
+#else
+    #error "Spinlocks are not supported"
+#endif
+}