Add RVV implementation for log

[ihb2032]

Add log operator implementation using RVV intrinsics.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 68.59504% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.78%. Comparing base (6089953) to head (fcf6fed).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layer/riscv/log_riscv_zfh.cpp	51.31%	37 Missing ⚠️
src/layer/riscv/log_riscv.cpp	97.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6638      +/-   ##
==========================================
+ Coverage   93.73%   93.78%   +0.05%     
==========================================
  Files         909      913       +4     
  Lines      286463   288299    +1836     
==========================================
+ Hits       268516   270383    +1867     
+ Misses      17947    17916      -31     

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[nihui]

Legacy layers from the Caffe era, whether in PyTorch or ONNX, will no longer convert to ncnn's Exp and Log, but instead use UnaryOp. However, since you've already optimized them, I believe necessary unit tests are essential. You need to add code like test_exp and test_log to your tests; you can refer to other test implementations for examples.

[nihui]

LGTM

[nihui]

Thanks for your contribution !