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

[Ahmed Bougacha via llvm-commits]

ahmedbougacha wrote:

> I just checked if there indeed are big-endian bots which should pick up if a different hash gets produced on a big-endian system. I guess this bot (the only bot?) would pick it up: https://lab.llvm.org/buildbot/#/builders/231

For whatever reason I can't find 231, but on clang-ppc64be-linux-test-suite/multistage, looks like the unittests are passing after all!

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