[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.

usr/testbed/test.cpp

@@ -1,7 +1,10 @@
 #include <mammoth/util/debug.h>
+#include <zcall.h>
 
 uint64_t main(uint64_t init_port_cap) {
   dbgln("testbed");
+  check(ZThreadSleep(2000));
+  dbgln("testbed2");
 
   return glcr::OK;
 }