x86 optimization for convolution int8 gemm

[nihui]

• conv int8 test 0.5 filtered
• coverage
• benchmark for avx2 avx512vnni xop

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.15668% with 4 lines in your changes missing coverage. Please review.

Project coverage is 95.07%. Comparing base (ace8372) to head (564cd1d).

Files with missing lines	Patch %	Lines
src/layer/x86/convolution_x86_avx2.cpp	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5874      +/-   ##
==========================================
- Coverage   95.10%   95.07%   -0.04%     
==========================================
  Files         824      824              
  Lines      277477   275571    -1906     
==========================================
- Hits       263904   261999    -1905     
+ Misses      13573    13572       -1     

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