Fixup build failure with clang-11.0.0

[ojab]

/tmp/ruby-build.20201111212834.9644.4pUxsX/rubinius-5.0/machine/diagnostics/timing.hpp:116:25: error: incompatible integer to pointer conversion assigning to 'std::atomic<uint64_t> *' (aka 'atomic<unsigned long> *') from 'uint64_t' (aka 'unsigned long')
      if(last_) last_ = run;
                        ^~~

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