Cycle counter for RISC-V for both rv32 & rv64, using 'rdtime' (original commit is using 'rdcycle', but direct userland access is to be prohibited).

'rdtime' might have a low resolution on some hardware, unfortunately (same principle as the two counters on Aarch64).

Those changes are placed in the public domain by their author Romain Dolbeau.

This is an alternative to #348.