Kernel+LibC: Implement clock_gettime() and clock_nanosleep()

Only the CLOCK_MONOTONIC clock is supported at the moment, and it only
has millisecond precision. :^)

Kernel/Thread.h

@@ -220,6 +220,7 @@ public:
     VirtualAddress thread_specific_data() const { return m_thread_specific_data; }
 
     u64 sleep(u32 ticks);
+    u64 sleep_until(u64 wakeup_time);
 
     enum class BlockResult {
         WokeNormally,