[llvm] [Support] Integrate SipHash.cpp into libSupport. (PR #94394)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I wonder whether the PAuth use case should use 128-bit xxhash instead (#95863), which is much faster.

64-bit xxhash is used by lld for string deduplication and clang for serialization.

https://github.com/llvm/llvm-project/pull/94394