Add xxh3 RISCV-V implementation from xxhash repo.

[BoBoDai]

This pr resolved #4471 .

Add RISCV-V implementation from xxhash repo.

Relevant performance tests are reflected in xxHash pr #1070 and #1066

I conducted testing by replacing the hash function in the fuzzer and conduct testing through qemu.

thread pool tests OK
1085 fuzzer tests completed

Although zstd does not currently use xxh3, this will better support teams who wish to use zstd and xxh3 on RISCV devices.

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[BoBoDai]

Hi, @Cyan4973 i copy the rvv xxh3 code from xxHash repo, and through fuzzer tests by qemu.
if you could take a look when you have a moment.
Thanks ：）

[Cyan4973]

This looks correct,
though also, as you noted,
it's not really useful, since zstd doesn't employ xxh3 anywhere.

(in fact, we even disable it explicitly to ensure it does not occupy binary space).

[BoBoDai]

Yes, I conducted the xxh3 fuzzer test by deleting this line.
In fact, repo like zstd-jni can provide Java with .so files that are quick and easy to use, which also relies on the zstd.
This undoubtedly provides an additional option for those who wish to use xxh3.