kernel/svc: Migrate svcCancelSynchronization behavior to a thread function

The actual behavior of this function is slightly more complex than what
we're currently doing within the supervisor call. To avoid dumping most
of this behavior in the supervisor call itself, we can migrate this to
another function.

src/core/hle/kernel/thread.h

@@ -164,11 +164,17 @@ public:
         return tls_memory;
     }
 
-    /**
-     * Resumes a thread from waiting
-     */
+    /// Resumes a thread from waiting
     void ResumeFromWait();
 
+    /// Cancels a waiting operation that this thread may or may not be within.
+    ///
+    /// When the thread is within a waiting state, this will set the thread's
+    /// waiting result to signal a canceled wait. The function will then resume
+    /// this thread.
+    ///
+    void CancelWait();
+
     /**
      * Schedules an event to wake up the specified thread after the specified delay
      * @param nanoseconds The time this thread will be allowed to sleep for