[RISC-V] Release-Acquire semantics for LR/SC

[hernanponcedeleon]

There seem to be some subtle issues with the Release-Acquire tags of LR and SC instructions in RISC-V (see this and this).

We need to be sure that we are up to date wrt this.

[ThomasHaas]

The issue is just that release tags on Loads and acquire tags on Stores are meaningless.
I think fixing this in the parser is the easiest solution. Keep in mind that this only affects litmus tests at best.