Add shuffle channel rvv optimization

[AtomAlpaca]

主要改动：

参考 #4907 添加了 shuffle channel rvv 优化
几乎没有对原 pr 的代码逻辑进行改动

close #6206

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.71%. Comparing base (9b91fe5) to head (c9075d1).
⚠️ Report is 2 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (9b91fe5) and HEAD (c9075d1). Click for more details.

HEAD has 26 uploads less than BASE

Flag	BASE (9b91fe5)	HEAD (c9075d1)
	31	5

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #6243       +/-   ##
===========================================
- Coverage   95.92%   90.71%    -5.21%     
===========================================
  Files         836      292      -544     
  Lines      264899    89584   -175315     
===========================================
- Hits       254098    81265   -172833     
+ Misses      10801     8319     -2482     

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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15648008	15648008	0 😘
armhf	6649020	6649020	0 😘
aarch64	9987736	9987736	0 😘

[Copilot]

Pull Request Overview

This PR adds RISC-V vector (RVV) optimization for the ShuffleChannel layer operation. It introduces optimized implementations for both fp32 and fp16/bf16 data types, leveraging RISC-V vector extensions to improve performance compared to scalar implementations.

• Adds RVV-optimized shuffle channel operations with support for different element packing sizes
• Implements specialized code paths for common group sizes (2, 4, 8) using vector gather operations
• Provides fallback to unpacked processing for complex group configurations

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 7 comments.

File	Description
src/layer/riscv/shufflechannel_riscv.h	Header file defining the RISC-V-specific ShuffleChannel class with forward declaration methods
src/layer/riscv/shufflechannel_riscv.cpp	Main implementation with RVV optimizations for fp32 and fp16/bf16 shuffle operations