snappy: optimize UnalignedCopy64 and IncrementalCopy for RISC-V (#3360)

* snappy: optimize UnalignedCopy64 and IncrementalCopy for RISC-V

Use RISC-V inline assembly (ld/sd) for 8-byte copy operations instead
of generic macro-based implementation.

Changes:
- UnalignedCopy64: direct ld/sd pair for 8-byte copy
- IncrementalCopy: 8-byte bulk copies when source/dest don't overlap

Performance improvement (direct function benchmark):
- Decompress compressible-256K: 728 MB/s -> 2205 MB/s (+203%)
- Decompress zeros-256K: 543 MB/s -> 1462 MB/s (+169%)

Tests: brpc_snappy_compress_unittest passed (7/7)

Signed-off-by: Xiaofei Gong <gongxiaofei24@iscas.ac.cn>
Signed-off-by: YuanSheng <yuansheng@isrc.iscas.ac.cn>

* snappy: add alignment check for RISC-V ld/sd optimization

Address Copilot review: ld/sd require 8-byte alignment. Add runtime
alignment check and fall back to memcpy/byte-copy for unaligned
addresses to avoid traps on implementations that don't support
misaligned access.

Signed-off-by: Xiaofei Gong <gongxiaofei24@iscas.ac.cn>
Signed-off-by: YuanSheng <yuansheng@isrc.iscas.ac.cn>

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Signed-off-by: Xiaofei Gong <gongxiaofei24@iscas.ac.cn>
Signed-off-by: YuanSheng <yuansheng@isrc.iscas.ac.cn>