[clang] [llvm] [RISCV] Add Spacemit X100 processor definition (PR #173988)

[Olaf Bernstein via cfe-commits]

camel-cdr wrote:

We got some benchmarks: https://camel-cdr.github.io/rvv-bench-results/spacemit_x100/index.html (vector FP throughput is still missing due to a bug in my code)

I'm not sure if `TuneDLenFactor2` is actually desirable.
The throughput of simple integer RVV instructions for LMUL=1 and LMUL<1 is the same, so you'd be wasting half of the performance defaulting to LMUL=1/2.
The permutation, comparison and shift instructions seem faster at LMUL=1/2 then LMUL=1, but they got linear scaling from LMUL=1/2 to LMUL=1, so you should get the same performance per element at LMUL=1.

https://github.com/llvm/llvm-project/pull/173988