backend: setup llvm about eikyo

[dannypsnl]

Path

1. use llvm-hs 9.0 directly, this is available on Hackage. However, llvm@9 is not available on m1(apple silicon)
2. use https://github.com/llvm-hs/llvm-hs for 12.0, but still cannot get the build correct
3. try fork and use llvm 14, still cannot work
4. the problem is Stack, for whatever reason, invokes gcc with the flag -std=c++14 which is nonsense

Anyway, this task has already taken me two days, and don't have any hope to solve it in a short time. Let's move on and generate C directly.