Add RVV implementation for exp

[ihb2032]

Add exp operator implementation using RVV intrinsics.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 68.64407% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.79%. Comparing base (3318da2) to head (877a97a).
⚠️ Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layer/riscv/exp_riscv_zfh.cpp	51.35%	36 Missing ⚠️
src/layer/riscv/exp_riscv.cpp	97.72%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6637      +/-   ##
==========================================
+ Coverage   93.75%   93.79%   +0.03%     
==========================================
  Files         907      913       +6     
  Lines      286651   288079    +1428     
==========================================
+ Hits       268761   270191    +1430     
+ Misses      17890    17888       -2     

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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 !