[lld] [LLD][COFF] Add support for ARM64EC import call thunks. (PR #107931)

[Martin Storsjö via llvm-commits]

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

Looks good overall. I'm not entirely following 100% the bits about exit thunks here (and how it is handled through MarkLive) though, but the code looks reasonable. Some questions on the writing of the thunk.

https://github.com/llvm/llvm-project/pull/107931