win*.mak: Sync compiler flags with Posix ones

[kinke]

Unfortunately, these files seem untested by CI here, but they are used by Azure pipelines for DMD, so one can easily break DMD CI by a seemingly fine Phobos change (#7142 in this case, which requires DIP1000). From:

phobos/posix.mak

Line 142 in a2b9977

override DFLAGS+=-conf= -I$(DRUNTIME_PATH)/import $(DMDEXTRAFLAGS) -w -de -preview=dip1000 $(MODEL_FLAG) $(PIC) -transition=complex

[dlang-bot]

Thanks for your pull request and interest in making D better, @kinke! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#7143"

[wilzbach]

They should be tested on auto-tester as well.