add sw_64 platform support

[wzyforgit]

Add "sw_64" platform support
All unit test have passed
Models that passed test: PaddleOCR v2.0, MobileNet v3, SqueezeNet v1.1
My CPU is "sw421"

[codecov-commenter]

Codecov Report

Merging #3421 (274a676) into master (878cb71) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3421      +/-   ##
==========================================
- Coverage   91.03%   91.03%   -0.01%     
==========================================
  Files         528      527       -1     
  Lines      145263   145213      -50     
==========================================
- Hits       132243   132193      -50     
  Misses      13020    13020              

Impacted Files	Coverage Δ
src/layer.cpp	46.00% <0.00%> (-1.06%)	⬇️
src/layer/arm/convolution_sgemm_pack8to1_int8.h	88.41% <0.00%> (-0.10%)	⬇️
src/layer/arm/convolution_sgemm_pack8to4_int8.h	98.22% <0.00%> (-0.03%)	⬇️
src/layer/arm/convolution_sgemm_pack1to4_int8.h	96.79% <0.00%> (-0.02%)	⬇️
src/layer/arm/convolution_sgemm_int8.h	98.43% <0.00%> (-0.01%)	⬇️
src/layer/arm/convolution_arm_arm82dot.cpp

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[nihui]

Thanks for your contribution !