[lld] [llvm] [lld][llvm/mc] Add support for sdata8 for FDE CIE (PR #174508)

[Fangrui Song via llvm-commits]

MaskRay wrote:

As suggested by Peter, the lld/test test is unrelated and should be removed. 
Yes, we can add a cl::opt (-mllvm -xxx) for such toggles. I think it's clearer to merge #174637 into this PR (after removing the lld change). #174637 should be gated by the cl::opt option, not change the existing `sdata4` behavior for CodeModel::Medium.

https://github.com/llvm/llvm-project/pull/174508