[lld] [LLD][COFF] Require explicit specification of ARM64EC target (PR #116281)

[Martin Storsjö via llvm-commits]

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

LGTM, I think this is good now. But let's give @dpaoliello a chance to get back and comment as well.

https://github.com/llvm/llvm-project/pull/116281