[RISCV] Avoid emitting hardware fences for singlethread fences

singlethread fences only synchronize with code running on the same hardware thread (i.e. signal handlers). Because of this, we need to prevent instruction reordering, but do not need to emit hardware fence instructions.

The implementation strategy here matches many other backends. The main motivation of this patch is to introduce the MEMBARRIER node and get some test coverage for it.

Differential Revision: https://reviews.llvm.org/D141311