unaryop avx512 mask optimization

[lfalive]

#6062

[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	16511232	16503040	-8192 😘
armhf	7369820	7369820	0 😘
aarch64	10775560	10775560	0 😘

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.70%. Comparing base (7fd167f) to head (1e98457).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6098      +/-   ##
==========================================
+ Coverage   95.59%   95.70%   +0.10%     
==========================================
  Files         827      827              
  Lines      270122   270128       +6     
==========================================
+ Hits       258232   258533     +301     
+ Misses      11890    11595     -295     

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[Copilot]

Pull Request Overview

This PR optimizes the unary operation implementation by leveraging AVX512 mask instructions to handle the remaining elements when processing vectors.

• Removed obsolete preprocessor guards for SSE2/AVX in the AVX512 code block
• Added a remainder handling branch using __mmask16 for handling elements not divisible by 16

[nihui]

Thanks for your contribution !