Switch to CLOCK_BOOTTIME for HSA system clock.

This is consistent with KFD and has significantly better latency.
KFD is taking this as the definition of the SystemClockCounter.

Change-Id: I4c1b3bc58c738206265c55ebefd41356c013bfe5

runtime/hsa-runtime/core/util/win/os_win.cpp

@@ -270,6 +270,16 @@ void DestroySharedMutex(SharedMutex lock) {
   abort();
 }
 
+uint64_t ReadSystemClock() {
+  assert(false && "Not implemented.");
+  abort();
+}
+
+uint64_t SystemClockFrequency() {
+  assert(false && "Not implemented.");
+  abort();
+}
+
 }   //  namespace os
 }   //  namespace rocr