[SystemZ][z/OS] z/OS does not support nanosleep, use usleep instead (#109823)

Use usleep instead of nanosleep to resolve a build error on z/OS because
there is no support for nanosleep.

Author: abhina-sree

llvm/unittests/Support/TimerTest.cpp

@@ -27,8 +27,13 @@ void SleepMS() {
   struct timespec Interval;
   Interval.tv_sec = 0;
   Interval.tv_nsec = 1000000;
+#if defined(__MVS__)
+  long Microseconds = (Interval.tv_nsec + 999) / 1000;
+  usleep(Microseconds);
+#else
   nanosleep(&Interval, nullptr);
 #endif
+#endif
 }
 
 TEST(Timer, Additivity) {