[llvm] [RISCV][llvm-exegesis] Add default Pfm cycle counter. (PR #121866)

[Craig Topper via llvm-commits]

topperc wrote:

> > Using repetition-mode=middle-half-duplicate|middle-half-loop both gave a value closer to 1.0 for simple scalar, and 3.0 for mul. I was using the default before. This is my first time running llvm-exegesis.
> 
> How much closer? On x86 they're pretty on the dot and can even reasonably consistently give integer throughputs. If that gives a value pretty close to one it's interesting how high the ioctl syscall overhead is.
> 
> I'm not sure it's a super common option. I implemented it I think about a year ago and assume I'm probably the only user currently.

With `middle-half-loop`, for ADD I got between 0.9761 and 1.0148. With `middle-half-duplicate` I got between 0.9839 and 1.0468.

https://github.com/llvm/llvm-project/pull/121866