[llvm] [RISCV] Adopt SpacemitX60's scheduling model for `-mtune=generic` (PR #167008)

[Petr Hosek via llvm-commits]

petrhosek wrote:

I tested this change on [Raspberry Pi Pico 2](https://www.raspberrypi.com/products/rp2350/) ([Hazard3 RISC-V core](https://github.com/Wren6991/Hazard3)) using [Coremark](https://github.com/eembc/coremark) and compared the performance of different scheduling models for `-mtune=generic-rv32`:

- With the `NoSchedModel` the score is 438.58.
- With the `SpacemitX60Model` the score is 428.58.
- With the `RocketModel` the score is 432.36.

These results show that on this particular core and with this particular benchmark the SpacemitX60 scheduling model regresses performance by roughly 2.3% over no scheduling model (the Rocket scheduling model regresses performance by roughly 1.4% over no scheduling model).

The compiler flags I used were: `--target=riscv32-unknown-elf -march=rv32imac_zicsr_zifencei_zba_zbb_zbs_zbkb -mabi=ilp32 -O3`.

Let me know if you'd like to me to try different flags or additional benchmarks.

https://github.com/llvm/llvm-project/pull/167008