Support simpleomp on Windows with MSVC

[Jiang-Weibo]

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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]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
0 out of 3 committers have signed the CLA.

:x: s2496214
:x: Jiang-Weibo
:x: nihui

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[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 95.60%. Comparing base (d2c1bd1) to head (9f04fba). :warning: 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[bot]]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15155464	15155464	0 :kissing_heart:
armhf	6182208	6182208	0 :kissing_heart:
aarch64	9520608	9520608	0 :kissing_heart:

[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!