Ongoing work for adding per-thread UTCB structures.

Added setting of utcb address to l4_thread_control.
This is going to be moved to exchange_registers() since we need to pass
both the utcb physical and virtual address and exregs fits such context
modification better than thread_control.

include/l4/api/kip.h

@@ -68,6 +68,8 @@ struct kip {
 	u32 arch_syscall2;
 	u32 arch_syscall3;
 
+	u32 utcb;
+
 	struct kernel_desc kdesc;
 } __attribute__((__packed__));