[Zion] Add a thread sleep call.

For now this can only sleep in increments of the scheduler quantum
(currently 50ms). It also uses a somewhat ineffecient way of tracking
the sleeping threads - it will scale linearly with the number of
sleeping threads.

zion/syscall/syscall.cpp

@@ -59,6 +59,7 @@ extern "C" z_err_t SyscallHandler(uint64_t call_id, void* req) {
     CASE(ThreadStart);
     CASE(ThreadExit);
     CASE(ThreadWait);
+    CASE(ThreadSleep);
     // syscall/address_space.h
     CASE(AddressSpaceMap);
     CASE(AddressSpaceUnmap);