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

[Jacek Caban via llvm-commits]

cjacek wrote:

With #151255, we also need entry thunk support for `SameAddressThunkARM64EC`. On top of this PR, that just requires overriding `getEntryThunk`. Before the recent change that moved the write logic, I also had to override `writeTo` to write the offset there (that’s not a big deal, so I can go back to that if preferred).

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