[lld] [lld] lld::makeThreadLocal: omit `thread_local` when threads disabled (PR #72057)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Can you describe how is your single-threaded environment look like? Is `thread_local` an error for your compiler, linker, or loader?

It seems that we don't use `thread_local` for `!LLVM_ENABLE_THREADS` configurations in `llvm/` and `clang/` (but not `mlir/`) and I don't think it is good for the project if we add a lot of code in non-testable configurations (those with very special requirements).

https://github.com/llvm/llvm-project/pull/72057