Added a stub for Syscall::Yield() which is used by broken code.

The whole system call interface is currently stupid.

sortix/syscall.cpp

@@ -83,6 +83,12 @@ namespace Sortix
 			Scheduler::Switch(regs);
 		}
 
+		void Yield()
+		{
+			Panic("Syscall::Yield() is not implemented because it caused "
+			      "instability and other issues.");
+		}
+
 		void AsIs()
 		{
 			system_was_incomplete = 1;

sortix/syscall.h

@@ -44,6 +44,11 @@ namespace Sortix
 		// that its return value shall be discarded.
 		void Incomplete();
 
+		// Call this prior to Incomplete() to signal that the scheduler should
+		// go run something else for a moment. The current thread will not be
+		// marked as blocking.
+		void Yield();
+
 		// For when you want the syscall exit code not to modify registers.
 		void AsIs();