Use STCK to get the CPU clock on s390x (#540)

AUTHORS

@@ -31,6 +31,7 @@ Kishan Kumar <kumar.kishan@outlook.com>
 Lei Xu <eddyxu@gmail.com>
 Matt Clarkson <mattyclarkson@gmail.com>
 Maxim Vafin <maxvafin@gmail.com>
+MongoDB Inc.
 Nick Hutchinson <nshutchinson@gmail.com>
 Oleksandr Sochka <sasha.sochka@gmail.com>
 Paul Redmond <paul.redmond@gmail.com>

CONTRIBUTORS

@@ -53,6 +53,7 @@ Pierre Phaneuf <pphaneuf@google.com>
 Radoslav Yovchev <radoslav.tm@gmail.com>
 Raul Marin <rmrodriguez@cartodb.com>
 Ray Glover <ray.glover@uk.ibm.com>
+Robert Guo <robert.guo@mongodb.com>
 Roman Lebedev <lebedev.ri@gmail.com>
 Shuo Chen <chenshuo@chenshuo.com>
 Tobias Ulvgård <tobias.ulvgard@dirac.se>

src/cycleclock.h

@@ -159,6 +159,11 @@ inline BENCHMARK_ALWAYS_INLINE int64_t Now() {
   struct timeval tv;
   gettimeofday(&tv, nullptr);
   return static_cast<int64_t>(tv.tv_sec) * 1000000 + tv.tv_usec;
+#elif defined(__s390__) // Covers both s390 and s390x.
+  // Return the CPU clock.
+  uint64_t tsc;
+  asm("stck %0" : "=Q" (tsc) : : "cc");
+  return tsc;
 #else
 // The soft failover to a generic implementation is automatic only for ARM.
 // For other platforms the developer is expected to make an attempt to create