Fix xxhash64 to support seeds larger than u32.

[creationix]

What does this PR do?

Hopefully fix #21879

How did you verify your code works?

Added a test with a seed larger than u32.

The test vector is from this tiny test I wrote to rule out upstream zig as the culprit:

const std = @import("std");
const testing = std.testing;
test "xxhash64 of short string with custom seed" {
    const input = "";
    const seed: u64 = 16269921104521594740;
    const hash = std.hash.XxHash64.hash(seed, input);
    const expected_hash: u64 = 3224619365169652240;
    try testing.expect(hash == expected_hash);
}

[RiskyMH]

can you make a test so we can verify that it fixes it!

[creationix]

ok, test added.

[RiskyMH]

nice, in ~1hr all the tests should have ran

also if it does work, the other ones prob should be updated too if needed - and then maybe in a different test part section testing for longer size

[Jarred-Sumner]

Thanks. Will merge once macOS ci finishes (it's a bit behind right now)