[litmus] riscv timebase and cache operations

[npigginTT]

No description provided.

[maranget]

Hi @npigginTT. I have tested your PR (with qemu). I have performed two useful additions

1. For C compilation to function I had to add a few options to gcc, (commit 318f82e adds a new configuration file riscv-qemu+cache).
2. For option -presi mode to function a small patch is needed (commit bad4b83).

If you have RISCV harware at your disposal, would you please test my modifications number two?
(Add -mode presi to your usual set of options).

Thanks