Merging r366868:

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r366868 | rogfer01 | 2019-07-24 07:33:46 +0200 (Wed, 24 Jul 2019) | 6 lines

[RISCV] Implement benchmark::cycleclock::Now

This is a cherrypick of D64237 onto llvm/utils/benchmark and
libcxx/utils/google-benchmark.

Differential Revision: https://reviews.llvm.org/D65142
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llvm-svn: 367813