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

[Anton Korobeynikov via llvm-commits]

asl wrote:

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

My understanding (@ahmedbougacha please correct me if I'm wrong) is that siphash is essentially a part of ABI on arm64e. And we can expect code to be shared between arm64e and other pauth-aware platforms expecting the same discriminator values.

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