feat(RISC-V): Add RVV-optimized implementation for memcopy64

[anthony-zy]

This PR add RVV to optimize Memcopy64 function , improving compression speed by ~46%.

Optimize Snappy 1.2.2 performance

Added RVV support for RISC-V in Snappy, optimizing Memcopy64 by leveraging RVV vector load/store instructions (e.g., vle8_v_u8m2, vse8_v_u8m2) to reduce memory copy overhead and improve decompression performance. lzbench 2.1 tests onsilesia.tar (GCC 13.2.1, 64-bit Linux) show:

Compressor	Compress	Decompress	Size	Ratio
Snappy 1.2.2 (Before)	64.3 MB/s	186 MB/s	101403263	47.84%
Snappy 1.2.2 (After)	64.4 MB/s	272 MB/s	101403263	47.84%

Snappy 1.2.2 unittest ([ PASSED ] 21 tests.)

Running main() from gmock_main.cc
[==========] Running 21 tests from 3 test suites.
[----------] Global test environment set-up.
[----------] 1 test from CorruptedTest
[ RUN ] CorruptedTest.VerifyCorrupted
Crazy decompression lengths not checked on 64-bit build
[ OK ] CorruptedTest.VerifyCorrupted (5 ms)
[----------] 1 test from CorruptedTest (5 ms total)

[----------] 17 tests from Snappy
[ RUN ] Snappy.SimpleTests
[ OK ] Snappy.SimpleTests (22 ms)
[ RUN ] Snappy.AppendSelfPatternExtensionEdgeCases
[ OK ] Snappy.AppendSelfPatternExtensionEdgeCases (3 ms)
[ RUN ] Snappy.AppendSelfPatternExtensionEdgeCasesExhaustive
[ OK ] Snappy.AppendSelfPatternExtensionEdgeCasesExhaustive (4830 ms)
[ RUN ] Snappy.MaxBlowup
[ OK ] Snappy.MaxBlowup (11 ms)
[ DISABLED ] Snappy.DISABLED_MoreThan4GB
[ RUN ] Snappy.RandomData
[ OK ] Snappy.RandomData (20311 ms)
[ RUN ] Snappy.FourByteOffset
[ OK ] Snappy.FourByteOffset (1 ms)
[ RUN ] Snappy.IOVecSourceEdgeCases
[ OK ] Snappy.IOVecSourceEdgeCases (0 ms)
[ RUN ] Snappy.IOVecSinkEdgeCases
[ OK ] Snappy.IOVecSinkEdgeCases (0 ms)
[ RUN ] Snappy.IOVecLiteralOverflow
[ OK ] Snappy.IOVecLiteralOverflow (0 ms)
[ RUN ] Snappy.IOVecCopyOverflow
[ OK ] Snappy.IOVecCopyOverflow (0 ms)
[ RUN ] Snappy.ReadPastEndOfBuffer
[ OK ] Snappy.ReadPastEndOfBuffer (0 ms)
[ RUN ] Snappy.ZeroOffsetCopy
[ OK ] Snappy.ZeroOffsetCopy (0 ms)
[ RUN ] Snappy.ZeroOffsetCopyValidation
[ OK ] Snappy.ZeroOffsetCopyValidation (0 ms)
[ RUN ] Snappy.FindMatchLength
[ OK ] Snappy.FindMatchLength (0 ms)
[ RUN ] Snappy.FindMatchLengthRandom
[ OK ] Snappy.FindMatchLengthRandom (1145 ms)
[ RUN ] Snappy.VerifyCharTable
[ OK ] Snappy.VerifyCharTable (0 ms)
[ RUN ] Snappy.TestBenchmarkFiles
[ OK ] Snappy.TestBenchmarkFiles (406 ms)
[----------] 17 tests from Snappy (26734 ms total)

[----------] 3 tests from SnappyCorruption
[ RUN ] SnappyCorruption.TruncatedVarint
[ OK ] SnappyCorruption.TruncatedVarint (0 ms)
[ RUN ] SnappyCorruption.UnterminatedVarint
[ OK ] SnappyCorruption.UnterminatedVarint (0 ms)
[ RUN ] SnappyCorruption.OverflowingVarint
[ OK ] SnappyCorruption.OverflowingVarint (0 ms)
[----------] 3 tests from SnappyCorruption (0 ms total)

[----------] Global test environment tear-down
[==========] 21 tests from 3 test suites ran. (26740 ms total)
[ PASSED ] 21 tests.

YOU HAVE 1 DISABLED TEST
[ PASSED ] 21 tests.

[anthony-zy]

@danilak-G
I've revised the code, reducing branches and optimizing preprocessor directives for easier review. Could you check the current version when possible?
Also, in Snappy's maintenance mode, what's the review timeline and what changes are typically accepted? Thanks!