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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0 out of 3 committers have signed the CLA.

❌ s2496214
❌ Jiang-Weibo
❌ nihui

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.60%. Comparing base (d2c1bd1) to head (9f04fba).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5683   +/-   ##
=======================================
  Coverage   95.60%   95.60%           
=======================================
  Files         840      840           
  Lines      265625   265625           
=======================================
+ Hits       253955   253957    +2     
+ Misses      11670    11668    -2     

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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15155464	15155464	0 😘
armhf	6182208	6182208	0 😘
aarch64	9520608	9520608	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!