Support simpleomp on Windows with MSVC

[Jiang-Weibo]

1. Modify the CMakeLists.txt on the top level to leverage openmp on MSVC
2. Support simpleomp on MSVC
3. Test the results using the existed tests folder

[tencent-adm]

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2 out of 3 committers have signed the CLA.

✅ Jiang-Weibo
✅ nihui
❌ s2496214

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.42%. Comparing base (8363040) to head (d400fd9).
Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #5683       +/-   ##
===========================================
- Coverage   95.16%   94.42%    -0.75%     
===========================================
  Files         743      464      -279     
  Lines      252233   133625   -118608     
===========================================
- Hits       240043   126175   -113868     
+ Misses      12190     7450     -4740     

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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	16465128	16465128	0 😘
armhf	7335212	7335212	0 😘
aarch64	10704240	10704240	0 😘

[Copilot]

Pull Request Overview

This PR enables support for simpleomp on Windows with MSVC by adding Windows-specific branches in the source code and updating the CMake and GitHub Actions workflow files to configure and test the MSVC build.

• Introduces _WIN32-specific typedefs, initialization, and task dispatch code in src/simpleomp.cpp.
• Adds new build and test steps in the Windows workflow to compile and run the simpleomp configuration.

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/simpleomp.cpp	Adds Windows/MSVC branches for OpenMP support and task dispatch.
.github/workflows/windows.yml	Introduces build and test steps for the x64-simpleomp configuration.

Files not reviewed (1)

• src/CMakeLists.txt: Language not supported

[whyb]

Hello @Jiang-Weibo ,

Thank you very much for your outstanding contribution to simpleomp.cpp! Your modifications have given me new insights into optimization, and I have also tried applying the changes from your PR to my project as a test case. The relevant code can be found here. If you have some time, could you take a look and see if there is room for improvement? Your professional insights would be incredibly valuable to me.

Once again, I really appreciate your great work, and I look forward to seeing more of your contributions in the community!