[lld] [LLD][COFF] Move entry thunk offset writing to writeSections (NFC) (PR #151254)

[Martin Storsjö via llvm-commits]

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

This is probably fine - it looks a bit unintuitive, but probably is good if it makes the following commit easier. However I don't quite see in which way this makes the later commit easier, as I don't see anything touching this area in that commit?

https://github.com/llvm/llvm-project/pull/151254