[test-suite] Fix RISC-V Support in benchmark 1.3.0

[lenary]

Summary

Fixed by backporting the upstream fix from here:

https://github.com/google/benchmark/pull/833

[lebedev.ri]

Looks like there is no MicroBenchmarks/libs/benchmark-1.3.0/README.llvm, unlike one in LLVM.
Could you please create one and document this cherry-pick?

[lenary]

Address comments:

• Add README.llvm and document benchmark cherry-picks

[lebedev.ri]

LGTM thank you!

test-suite/trunk/MicroBenchmarks/libs/benchmark-1.3.0/README.llvm

+LLVM notes
+----------
+This directory contains the Google Benchmark source code. Currently, the checked out
+Benchmark library version is v1.3.0.
+
+This directory is under a different license than LLVM.
+
+Changes:
+* https://github.com/google/benchmark/commit/ff2c255af5bb2fc2e5cd3b3685f0c6283117ce73
+  is applied on top of v1.3.0 to add s390x Support.
+* https://github.com/google/benchmark/commit/aad6a5fa767529d3353bd3beb89e126c7b0868ca
+  is applied on top of v1.3.0 to add NetBSD Support.
+* https://github.com/google/benchmark/commit/4abdfbb802d1b514703223f5f852ce4a507d32d2
+  is applied on top of v1.3.0 to add RISC-V timer support.

test-suite/trunk/MicroBenchmarks/libs/benchmark-1.3.0/src/cycleclock.h

#elif defined(__mips__)
   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;
+#elif defined(__riscv) // RISC-V
+  // Use RDCYCLE (and RDCYCLEH on riscv32)
+#if __riscv_xlen == 32
+  uint64_t cycles_low, cycles_hi0, cycles_hi1;
+  asm("rdcycleh %0" : "=r"(cycles_hi0));
+  asm("rdcycle %0" : "=r"(cycles_lo));
+  asm("rdcycleh %0" : "=r"(cycles_hi1));
+  // This matches the PowerPC overflow detection, above
+  cycles_lo &= -static_cast<int64_t>(cycles_hi0 == cycles_hi1);
+  return (cycles_hi1 << 32) | cycles_lo;
+#else
+  uint64_t cycles;
+  asm("rdcycle %0" : "=r"(cycles));
+  return cycles;
+#endif
 #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
 // a fast implementation and use generic version if nothing better is available.
 #error You need to define CycleTimer for your OS and CPU
 #endif
 }
 }  // end namespace cycleclock
 }  // end namespace benchmark
 
 #endif  // BENCHMARK_CYCLECLOCK_H_