RISC-V RDCYCLE cannot be accessed from userland

Starting with Linux kernel 6.6, the RDCYCLE instruction for RISC-V cannot be used with userland:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc4c07c89aada16229084eeb93895c95b7eabaa3

We use this in `my_rdtsc.h` for RISC-V platforms, which means this code does not work any more.

Instead we should use RDTIME, which while less accurate has the advantage of being synchronized between CPU (and thus monotonic) and of constant frequency. This exists in earlier kernel versions too, so will not break backwards compatibility.