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cycleclock: Fix type conversion to match function return type on riscv64 (#1802)

Fixes builds with clang

src/cycleclock.h:213:10: error: implicit conversion changes signedness: 'uint64_t' (aka 'unsigned long') to 'int64_t' (aka 'long') [-Werror,-Wsign-conversion]
     213 |   return cycles;
         |   ~~~~~~ ^~~~~~
   1 error generated.