[llvm] [MC][RISCV][LoongArch] Add AlignFragment size if layout is available and not need insert nops (PR #76552)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

Thanks for the patch. LGTM once the `## TODO Handle alignment directive.` comment is fixed and `fixups-expr.s` check prefixes switch to `-NEXT` (I'll change the existing `fixups-expr.s` check prefixes to use `-NEXT`)

https://github.com/llvm/llvm-project/pull/76552