rvs-profile: Replace A with Zalrsc

[darius-bluespec]

There has been some concern that the atomic extension (A) is too burdensome for certain soft CPU designs. In lieu of the A extension, it is possible to only implement the LR/SC instructions (Zalrsc). However, at the moment, some target software (e.g., Linux, glibc) make active use of AMO instructions. Thus, that software would need to be modified to use LR/SC sequences in place of AMO instructions, or the AMO instructions would need to be provided by trap and emulate in supervisor or machine mode.

In any case, I think it is important to show a concrete proof of concept working system with Zalrsc instead of A before the specification reflects the reduced requirement.