[LLD][COFF] Follow up comments on pr146610 (#147152)
This is a follow-up PR for post-commit comments in
https://github.com/llvm/llvm-project/pull/146610
- Changed "exporteddllmain" references to "importeddllmain".
- Add support for x86 target and test coverage.
- Changed a comment to better express why we're skipping importing
`DllMain`.
