[clang] [llvm] [RISCV] Add processor definition for XiangShan-KunMingHu (PR #89359)

[Camel Coder via llvm-commits]

camel-cdr wrote:

Has the target architecture been finalized? (As in what it should be, not necessarily the rtl)

Just yesterday, there was a significant change in vector execution units:

[before](https://github.com/OpenXiangShan/XiangShan/blob/e25e4d90505c592524b410b127fe611ac49a3adf/src/main/scala/xiangshan/Parameters.scala#L355):
```
VFEX0: VfaluCfg, VfmaCfg, VialuCfg, VimacCfg
VFEX1: VipuCfg, VppuCfg, VfcvtCfg, F2vCfg, F2fCfg, F2iCfg, VSetRvfWvfCfg
VFEX2: VfaluCfg, VfmaCfg, VialuCfg
VFEX3: VfdivCfg, VidivCfg
```
[after](https://github.com/OpenXiangShan/XiangShan/blob/2e61107/src/main/scala/xiangshan/Parameters.scala#L357):
```
VFEX0: VfmaCfg, VialuCfg, VimacCfg, VppuCfg
VFEX1: VfaluCfg, VfcvtCfg, VipuCfg, VSetRvfWvfCfg
VFEX2: VfmaCfg, VialuCfg, F2vCfg
VFEX3: VfaluCfg, VfcvtCfg
VFEX4: VfdivCfg, VidivCfg
VFEX5: VfdivCfg, VidivCfg
```


https://github.com/llvm/llvm-project/pull/89359