riscv: add prefetch optimization for doubleFast compressor

[Felix-Gong]

Summary

• Add RISC-V 64-bit prefetch optimization to zstd doubleFast block compressor
• Extend existing aarch64 PREFETCH_L1(ip+256) to RISC-V via macro guard
• 2 lines changed in zstd_double_fast.c

Changes

Extended the architecture guard from #if defined(__aarch64__) to #if defined(__aarch64__) || (defined(__riscv) && (__riscv_xlen == 64)) in two locations:

1. ZSTD_compressBlock_doubleFast_noDict_generic (line 236)
2. ZSTD_compressBlock_doubleFast_dictMatchState_generic (line 451)

Performance Results (RISC-V 64-bit, RV64GC)

Data Type	Compress	Decompress
Random (100MB)	+9.0% (337.3 → 367.6 MB/s)	+8.0% (2763.0 → 2983.9 MB/s)
Zero (100MB)	-0.3% (within noise)	-0.05% (within noise)

Testing

• Platform: RISC-V 64-bit server
• Compiler: GCC 15.1
• Test data: Random and zero-filled data (100MB each)
• Benchmark: zstd -b

Notes

• The prefetch targets sequential data access patterns in the compression loop
• No additional computation added to the hot loop
• Follows the same pattern as the existing aarch64 optimization

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