add riscv rvv support for lstm operator

[chenglimin]

all test cases passed on musepi and k230.

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✅ chenglimin
❌ nihui
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[chatgpt-codex-connector]

Add non-RVV fallback for LSTM_riscv implementation

lstm_riscv.cpp unconditionally uses RVV intrinsics (__riscv_vsetvl*, vfloat32m8_t, etc.) without any #if __riscv_vector guard, so generic RISC-V builds that compile baseline sources with -march=rv64gc (the default in toolchains/riscv64-linux-gnu.toolchain.cmake) fail to compile this layer. In this repo, cmake/ncnn_add_layer.cmake always adds src/layer/riscv/lstm_riscv.cpp for RISC-V and only generates a separate _rvv variant for vector builds, so the base file must either provide a scalar path or delegate to LSTM::forward when __riscv_vector is unavailable.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.26%. Comparing base (1b847b1) to head (4486cf9).
⚠️ Report is 118 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (1b847b1) and HEAD (4486cf9). Click for more details.

HEAD has 30 uploads less than BASE

Flag	BASE (1b847b1)	HEAD (4486cf9)
	32	2

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #6736       +/-   ##
===========================================
- Coverage   93.16%   87.26%    -5.91%     
===========================================
  Files         847      350      -497     
  Lines      266225   121443   -144782     
===========================================
- Hits       248040   105974   -142066     
+ Misses      18185    15469     -2716     

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