[Support] Import SipHash c reference implementation.

This brings the unmodified SipHash reference implementation:
  https://github.com/veorq/SipHash
which has been very graciously licensed under our llvm license
(Apache-2.0 WITH LLVM-exception) by Jean-Philippe Aumasson.

SipHash is a lightweight hash function optimized for speed on short
messages.  We use it as part of the AArch64 ptrauth ABI (in arm64e and
ELF PAuth) to generate discriminators based on language identifiers and
mangled names.

This commit brings the reference implementation (siphash.c@f26d35e)
unmodified, as SipHash.cpp.

Next, we will integrate it properly into libSupport, with a wrapping API
suited for the ptrauth use-case.