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 99.25651% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.83%. Comparing base (4a70be4) to head (01e3c91).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layer/x86/requantize_x86.cpp	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5874      +/-   ##
==========================================
+ Coverage   94.59%   95.83%   +1.23%     
==========================================
  Files         648      762     +114     
  Lines      219000   238772   +19772     
==========================================
+ Hits       207172   228826   +21654     
+ Misses      11828     9946    -1882     

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