Fix compiler warning (#706)

At compile time, gcc complains with:
../riscv/processor.cc:787:94: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 5 has type ‘uint64_t {aka long unsigned int}’ [-Wformat=]

The variable 'bits' is an uint64_t, so that PRIx64 should be used to print it out.