CMake: Force enable MSVC multi-processor build (/MP)

[whyb]

Force enable MSVC multi-processor build (/MP)

[tencent-adm]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.76%. Comparing base (d395000) to head (fff4eb0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6191      +/-   ##
==========================================
- Coverage   95.77%   95.76%   -0.01%     
==========================================
  Files         835      835              
  Lines      265103   265103              
==========================================
- Hits       253908   253887      -21     
- Misses      11195    11216      +21     

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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15606928	15606928	0 😘
armhf	6615712	6615712	0 😘
aarch64	9987680	9987680	0 😘